Selphie and Zell's first contact is very explosive and premonitory of their future relationship: depending on the player's options during the Dollet mission, Zell may start off by immediately disagreeing with her when she comments it would have been easier to jump off the cliff rather than walk the road, in which case she gets back at him by very maturely calling him a "chicken-wuss", then a pig, and then a chicken again.
The one thing to note here is that Zell is, in a lot of ways, Selphie's male counterpart: similarly to her, he is hyper and wild, which causes them to inevitably clash practically every time they speak to each other. Because, contrary to laid-back Selphie, Zell is extremely nervous, he usually reacts violently, leading to otherwise unlikely angry responses from her; not only do they rarely agree on anything, there's a particularly fun scene in his room when Balamb Town is under Galbadian occupation: although Zell has warned the other two party members to leave things as they are, Selphie, if she happens to be there, simply ignores him and happily jumps onto his bed, making him very mad. Indignant, she stands up, claims he is "anal", and, depending on the player's dialogue options, can either tell him he should "thank HER and feel honored that a very, very cute girl sat on his bed" or just go immediately back to where she was, lying on his bed, and lightly comment that he "doesn't understand anything", either way further frustrating him.
Despite all the arguments and name-calling, Zell and Selphie don't actually hate each other: their personalities and opinions may collide most of the time, but they support each other as SeeDs and would probably have each other's back upon external threat with no hesitation (well, she does run to help save him from choking on hot dog in the ending FMV...). Although neither of them would admit it, deep down they are friends.