It shouldn't be too hard, since you've only just gotten this tail. Up until Kid arrived in Vatheon, Rin never brushed his tail.

[She picks up a heavy-looking comb and sets to work raking out the biggest tangles.]

...[Not too hard, though.] I had a tail once, though. Brushing it wasn't that easy. It seems a lot more sensitive than hair usually is, doesn't it?