The room in front of hers has a little person living in it, assumed to be her little brother/sister. After the ending, your sibling gives you a red book, which is identical to the one in Irotsuki's room and allows you to explore a couple more worlds within your dream.
Irotsuki has short sandy blonde hair and wears a red hoodie with long black sleeves, a blue skirt and brown shoes. Her eye color is unknown and changes within fanart as well. The equipment screen is unavailable in the menu for this game but the option has been renamed "layer" to fit the art theme.
As a silent protagonist, there's not much to find out about Irotsuki's personality. Judging by the fact she lives alone with her younger sibling and they're seen playing together in the ending, you could describe her as a caring person. This fact is proved when you have to go all the way to a different world just to get medicine for the woman in the hospital bed.
Not much is known about her past, but she seems to be close to her sister, and could possibly be searching for inspiration to paint, similar to Alex.