You can build most of the personality. The thing is, the family's father passed away due to disease and they were left to live with their mother. Both of them are considerate of their mother but Isamu somewhat different towards others. The relationship between them as of recent mention are good. They both show respect towards each-other. There's this one rule though: Jirou is not allowed to leave the house after sunset without Isamu. That's all lol.
I'm okay with you volunteering but I'm hoping for more and choose someone I'm familiar with. Not that I don't want you to play Jirou but you did mention that you already have two active characters and three could be hard to manage. We'll see if anyone else volunteers and if they don't, you choose whether or not you want the spot.