Bha is the voiced aspirated labial, breathy voiced, and the fourth letter of the labial class. It is frequent in Sanskrit, standing at the head of the words for the lord and for devotion and of the large families of terms built on them, so a devotional or doctrinal text written in Siddham meets it constantly.
Like the other three voiced aspirates it has a character of its own rather than a mark added to the plain voiced stop, which keeps the fourfold structure of the class visible on the page and is part of why the Indic tables were copied rather than adapted when they reached East Asia.
It stands fourth in the labial class and is therefore the last of the four voiced aspirates in the table. With the nasal that follows it, the twenty-five stops are complete, and what comes after belongs to a different part of the analysis.
In the Japanese esoteric schools the letter carries a further role. The seed syllable of Shakyamuni is built on it, with the two dots of the visarga written after, and that syllable is what stands for the Buddha where an image, a memorial tablet or a mandala names a figure by letter rather than by name or by picture.
The gap between the letter and the syllable built on it is worth keeping in view. What stands on an image is a written syllable, a letter plus a mark, and treating it as a bare character misdescribes both the writing and the assignment. The reference charts of the Japanese schools record syllables, not letters.