There is the traditional story told that Usui saw the symbols appear in "a bubble of light". The symbols used in the Usui healing system are part of Taoist and Buddhist practice. It is believed that Usui, who had a background in Buddhism, simply adapted common Buddhist and other cultural/religious symbols for use in his system.
I personally do not believe the symbols are secret, although there are some Masters that do. People who identify themselves as "Traditional" Reiki Masters will generally not show the level II symbols to anyone not attuned to that level. This is an issue of contention for some in Reiki. Takata taught that the symbols were not to be openly shown. It is said that she would take the paper copies that her students wrote as practice and would burn them after the class. That was a great reverence for them. The symbols were first released in a book in Australia. They were next shown in Diane Stein's book, "Essential Reiki". The decision to show them or not, is an individual decision and there are people of good conscious on both sides of this. To argue regarding this is generally not productive as you are arguing from positions of belief and conditioning. What follows in this section is information regarding the symbols, their addition into Usui's practices, and information regarding how they are seen and used currently in Japanese Practice.
In the western "Traditional" practices the symbols are "secret" and "sacred". In the western "Non-Traditional" practices, it is not uncommon for them to be openly displayed. In Japan, the symbols appear to be somewhat common and have been observed to be written on all sorts of objects in public view. They are also a part of traditional Buddhist practice and are known by those of those traditions. It is said that in Usui's group, the "Usui Reiki Ryoho Gakkai", that the symbols are known but not used. There would therefore appear to be many different practices and opinions regarding the symbols, and different lineage's of reiki seem to have evolved their own ideas regarding this.
Some teach that the symbols have their own power, are tangible manifestations of source and all sorts of other beliefs. Some teach that seeing the symbols prior to being attuned will make it impossible to attune you. Some fear that exposing them to the public will degrade them or allow them to be used for harm or be desecrated. The symbols do not hold any "secret power", and have no power themselves. They are a convenient connection to the energy. The Symbols are a tool used to focus the Reiki energy. In order for these symbols to work you must be attuned to them. What happens during the attunement is that you are given the connection with the energy through them and they become a mental representation that helps you focus with them. In any case, Reiki cannot be used for harm. Seeing the symbols before being attuned is irrelevant. Countless people have seen them and been attuned without problem or incident. The symbols cannot be used for harm. The symbols have only the power you give to them. If you embody them with your power and belief, then they will have whatever power you imagine of them.
In American Reiki Practice, the tradition was an oral tradition with the symbols not ever being shown, and not written down. I have heard stories of masters having students practice writing the symbols in a level II class and then taking all copies of them and shredding and burning them. I have heard that showing them will make them loose their "power". I have heard that seeing them prior to an attunement will make it harder to attune the person later and all sorts of other beliefs. Symbols are a representation of something, not the thing itself. These may represent the connection to the Reiki energy, but they are not the source or the energy.
The Reiki symbols were not part of the beginning teachings by Usui but were incorporated into the reiki system some time before the system was taught to Hayashi Sensei. The symbols were originally referred to as Symbol 1, Symbol 2, Symbol 3, and Symbol 4, and the names were the mantras associated with them. The symbols were designed to assist those who had little or no energy experience so that they could more easily connect and use the system. The symbols were Reiki "training wheels", and once you had a strong connection to the energy they were no longer needed.
In the western Usui Shiki Ryoho of the Takata line the symbols are the primary basis of the system. In some cases they are considered secret and sacred. In recent times, Some independent masters have allowed the symbols to be viewed publicly.
I believe that there is a great deal of confusion about what symbols are, what they are for and why they are used. Many people get caught up on the outward form of them, that is the drawing, which symbol is correct in shape, etc. There is an oft repeated myth that Mrs. Takata gave the symbols differently to different masters. An interesting myth. However, I think that the reality is more likely that they remembered them differently. Mrs. Takata required an oral tradition. Takata took their reproductions that they drew in class and after class burned them. These were americans learning Japanese Kanji symbols from memory so I think it more likely that the difference in symbols that have crept into Reiki are more the result of human memory and limitations than them being taught differently. In any case, the differences in the outward form do not matter and they will all certainly work. Please do not misunderstand symbols, what they are, what they are for and why they are used. To concentrate so on the outward form misleads away from the inward transformation that they are.
At their simplest level they are a stimulus-responce tool. The mere act of drawing or visualizing them (stimulus) leads to connection with the energy (response).
It is said that Usui adapted them for use for those without the energy sensitivity to work with the energy directly. But again this should be placed in context. In the Usui Reiki Ryoho Gakkai the students would meet frequently (perhaps weekly). They would say the poetry of the meiji emperor. They would chant the 5 principles. They would do the hatsurei ho meditation to strengthen the channel and increase sensitivity. They would then receive an empowerment/attunement called reiju. They practiced healing. You can see from this description that one would develop the sensitivity relatively quickly and the repeated empowerments, hatsurei ho and other practices would lead to great familiarity with the energy. Hayashi changed this model to one where people learned more quickly. Empowerments became attunements. You did 4 at the first level, probably to reflect the many Usui did, and you worked in his clinic to gain experience. The symbols became more important because the community support and group work and empowerments were not the same.
In the west you don't get that kind of community experience.
It has been said that the symbols are reiki training wheels. That is they are used while needed and then let go, like all conditionings. But even with that said, the symbols were not chosen randomly. Usui would apparently have chosen symbols present in his environment that would have had meaning to himself and those around him. Some of that meaning is culturally laden.
Here is an excerpt from the "Reiki and Spirituality" article on this web site that I believe is worth repeating.
"....Usui was a grew up studying Tendai Buddhism. (For more information on Tendai (T'ien T'ai in Chinese) please visit the Reiki Ryoho Plain and Simple e-book. As such, there are certain ideas about the world that are part of his system. Reiki exemplifies the principle of "wei wu wei", doing by not doing. By doing nothing, everything is done. Reiki is a simple practice. We place our hands on someone and by doing nothing (no controlling, no directing, just BEing) everything is done. The principles teach keeping the now, and awareness. It takes mindfulness to "not anger" and "not worry". It takes mindfulness to "be grateful", and to "be kind to all living things".
Looking at the symbols he chose to place in the system also are keys to this enlightenment system. Let us now examine these symbols and the possible implications of them. Unlike the multitude of also ran systems that have developed in the west since the death of Mrs. Takata, the Usui system had only 4 symbols, and it is fairly clear that they were added fairly late in the development of the system. They seemed to be added in as a tool for those who had trouble connecting to the energy directly.
The first symbol is the "Cho Ku Rei", the so-called "power" symbol, also called the Focus. It says "place the power here". It is an edict to do so, but something much more is implied. It is also an edict to "wake up". In the oneness you are and always have been this, interconnected with all things and one. It is an invitation to awaken, to remember by releasing the things that blind us to our true nature.
The second symbol is the "Sei Hei Ki", or the so-called "mental-emotional" symbol. It is the one mind, Buddha Mind. It is the mind of compassion. It is the focus in the now where we are a new creation every moment. It is the Harmony symbol. In the oneness that is Cho Ku Rei, Sei Hei Ki heals body mind and spirit. It cleanses and heals by helping us to remove the attachments we have that create our suffering and thus our illness.
The third symbol is the "Hon Sha Ze Sho Nen", often called the "distant" symbol but also the "connection" symbol. It is from a Buddhist Chant that means "Right consciousness is the root of everything" or "A righteous man may correct all thoughts", or "To act in the Realization of Absolute Beingness". But what might that mean, "Right consciousness"? This is clearly a Buddhist idea. Right consciousness is being fully in the moment, fully in the now. It is acting, not reacting. This is an important distinction. Reacting is based on our ego, our conditionings. Our conditionings are the cause of our suffering and our illness. It is only by letting go and removing our conditionings that we are free to be in the moment, to respond, and ultimately to heal.
The last symbol is the "Dai Ko Mio", or the so-called "mastery" symbol. The symbol is the great shining light. The goal is Reiho is to live in the great shining light. The previous three symbols are the key, the last is the realization.
Perhaps putting them all together into a sentence might help. We are one with all things, in this oneness we heal body mind and spirit. In right consciousness we release attachments and conditionings so we live in the moment of the great shining light.
The symbols are a tool. They are a road map. They are training wheels on a bicycle that are used while needed and then removed, (like all conditionings) when no longer needed. Do not mistake the map for the journey, or the destination.