Why should you two be judged for who your birth father happens to be?
[It's not a full answer, but it's part of the reason. He hates how much family matters within the Order.
As for the rest of Yukio's questions...]
Mephisto was only in the paperwork as a technicality. I hadn't expected to die so soon, I had thought I would at least live long enough to see you two become adults.
Besides that, Mephisto is my oldest friend. I know it seems strange to you, but he's someone I trust not to kill the two of you.
[Mephisto is a demon, so Shiro is very careful with what he trusts him to do. But making sure Rin and Yukio are alright is one of those things, since it happens to be in his best interests too.]
Action
[It's not a full answer, but it's part of the reason. He hates how much family matters within the Order.
As for the rest of Yukio's questions...]
Mephisto was only in the paperwork as a technicality. I hadn't expected to die so soon, I had thought I would at least live long enough to see you two become adults.
Besides that, Mephisto is my oldest friend. I know it seems strange to you, but he's someone I trust not to kill the two of you.
[Mephisto is a demon, so Shiro is very careful with what he trusts him to do. But making sure Rin and Yukio are alright is one of those things, since it happens to be in his best interests too.]