Kshitigarbha, known in Japan as Jizo, is among the most widely honoured figures in Japanese Buddhism, and the small stone figures of him stand at roadsides, at crossings, at temple gates and in cemeteries in very large numbers. Where such a figure or its base carries writing, the writing is frequently a single Siddham letter, ka, which is his seed syllable.
That practice, of standing a whole figure on one letter, is general in the Japanese esoteric schools and not particular to this one bodhisattva. A syllable is assigned to a figure, and the syllable can then take the figure's place: written on an image, on a memorial tablet, on a ritual implement or on a mandala, it names who is meant without a picture and without a word. The reference charts that record which syllable belongs to which figure are a standard part of the shittan literature.
Two things follow that are worth stating carefully. The first is that this is a description of what is written, not of what is done with it. The contemplative and ritual uses these syllables belong to in Shingon and Tendai are transmitted by initiation, within a teacher relationship, and are not something the atlas sets out. The second is that the letter is not an ornament and does not become one by being legible: on a roadside stone it is a dedication, made by someone, for someone, and it is still doing that work whether or not the person passing can read it.