Obsessive

I see, you'd like to write about someone who is very blindly devoted. Someone who will do anything for their senpai. If it makes their Senpai happy, then they're happy. Nothing in the entire universe matters more than Senpai. If something pleases Senpai, it's holy. If something displeases Senpai, it must die.

The only thing they can't do for their senpai is make themselves sad, because following Senpai's orders makes them happy, no matter what the order is.