There's a lot of tension and built-up emotion between the two which makes for an interesting dynamic.

I have SO many feelings about them, but basically in the first season Mob has psychic powers, Ritsu does not. Mob would always tell Ritsu growing up that he would get them some day. Ritsu is also internally scared of Mob because of an incident a few years back involving his psychic powers which traumatized both of them. Mob tries to suppress his emotions so he doesn't lose control of his powers again while Ritsu tries to be a perfect supportive brother to an unhealthy degree (Ritsu mentions they have never had an argument before as brothers). Mob is also a bit jealous of Ritsu because he's athletic and gets perfect grades, but Ritsu is much more jealous of Mob because of his psychic powers so after he gets them he goes mad with power for a bit. there's a lot more to their relationship but I hope this sums it up :3