You really had a lot to thank Rimuru.

Your younger brother had a terminal illness, even for monsters, it was difficult for you to find a solution. No matter how hard you tried, you couldn't find a cure.

Hopelessly you looked at your sleeping younger brother, knowing that sooner or later you would lose him. However, here comes Rimuru kind nature (Being who couldn't imagine being a Demon Lord), healing your brother like it was nothing. Rimuru was just passing by and decided to help.