Koiwai is friends with Yanda, and often has him over at his house where Yanda eats instant ramen.
Yanda is considered Yotsuba's enemy, but he shows no sign of actually disliking her. His teasing is most likely just for fun. Yotsuba always has a negative attitude when she spends time with Yanda, and though Yanda initially seems to very much enjoy being a nuisance to Yotsuba, he later tries to be nicer. Whenever that happens, Yotsuba expresses surprise, but she still seems to see him as a rival.
Though Yanda, Koiwai and Jumbo are friends and hang out together, Jumbo seems to be less fond of Yanda than Koiwai is. For example, Jumbo calls Yanda "no good" for not helping Koiwai move and when the three of them go to the ranch together and Yanda says the cows aren't cute because they're so big Jumbo takes this as a personal insult, saying "why can't they be huge and cute?". He also encourages Yotsuba to "go beat" Yanda, when she tells him she is going to.
Yanda was, like Jumbo, amazed at Asagi's looks the moment he saw her, and is attracted to her which triggers Jumbo's rivalry. Yanda has a girlfriend of his own, Miki-chan, but she has never been seen and Koiwai assumes that she has broken up with Yanda because she has not been answering his calls for months. That does not stop Yanda from trying to get somewhere with Asagi though, and when Jumbo tells him that Asagi has a boyfriend he replies "that doesn't matter", which amazes Jumbo.