Hokuei's Lantern Ghost of Oiwa
 

What is the character on Oiwa's forehead in the rendition by Hokuei? What is its significance?

I did some searching, but haven't found anything about that.

I'm sorry.

Maybe someone else here knows a better way to search for information, better search terms?

It`s supposedly a Sanskrit character - or rather something that is supposed to look like Sanskrit.
Sanskrit has traditionally been used in connection with Buddhism in Japan for over 1000 years.

The character is a type of sanscrit called bonji (梵字), or more correctly, in the style of bonji. It symbolizes the Amida Buddha (Amitābha) who is associated with salvation of the dead.

Actually, the bonji character is a bit incomplete, which may indicate Oiwa's inability to "jobutsu suru;" ie, leave this earthly plane and move on.