Some Reiki lineages use a symbol called Raku, often described as a kind of grounding symbol. There's no Raku in Shingon Reiki, and it doesn't go back to Usui either. The name is still interesting, though, because the word raku can stand for more than one Japanese character. That's what this article is about, and a little bit about calligraphy too, where a character like 楽 really comes to life.
Not one of Usui's symbols 源
The symbols that go back to Usui himself are Cho Ku Rei, Sei He Ki, Hon Sha Ze Shō Nen and Dai Kō Myō. Raku isn't one of them. As far as I know, it was added later in some Reiki lineages, and I don't know of any Japanese source that mentions Raku in connection with Usui. So I don't work with it in Shingon Reiki.
If you're interested in the symbol that's often linked with the mental level, the answer is Sei He Ki. Its original form is the Siddham seed syllable (Siddham is called Bonji in Japanese) hrīḥ, the syllable of Amida and Senju Kannon. I explain how I came across this in detail in the article on Sei He Ki.
Two possible characters 楽 / 洛
Since Raku has only come down to us in Latin letters, we don't know which character might be meant. Japanese has several characters that are read raku. Two of them are the obvious candidates.
You could build a nice interpretation out of both meanings, something like "the joy of arriving at the center." But that would be a bit like playing the lottery. As long as no Japanese source shows which character is meant, any interpretation like that stays a guess. Strictly speaking, we can't even be sure the name goes back to a Japanese word at all.
The character 楽 in calligraphy 書
The character 楽 gets really interesting when you look at its old form 樂. According to the classical Chinese explanation, it shows musical instruments on a wooden stand, with the character for tree 木 at the bottom. From music came the joy it brings. Put simply, the character for ease originally holds a sound.
I wrote characters like this with the brush every day while I was a student in Heidelberg, with the Zen monk Hoon Kenmyo. In my very first lesson he handed me his own brush, and I had the feeling that the brush was guiding my hand rather than the other way around. That's why Japanese calligraphy (shodō 書道) is about more than beautiful shapes. The character carries the state of the person writing it. You'll find more on this in the article on spiritual calligraphy.
Raku ware 楽焼
In Japan the word Raku also turns up in a completely different context, in Raku ware, raku-yaki 楽焼. This pottery was created in the 16th century in Kyōto for the tea ceremony of Sen no Rikyū. The bowls are shaped by hand, without a potter's wheel, and taken out of the kiln while they're still glowing. Here the character 楽 is clearly the one meant. Historically, this pottery has nothing to do with a Reiki symbol, but it's a lovely example of how much can be packed into a single Japanese word.
Which character would you like to write with a brush yourself one day?