Fenris’ is actually the only rivalmance I’ve done, though that was a long time ago on my first playthrough (don’t ask the details, it was a mess getting there lmao. Let’s just say Hawke slept with 3 people and I had to make alt save files to keep from feeling like a horrible person). So I can speak to it a bit, though my memory sometimes fails me lmao.
But yeah, it’s not pleasant, though I’d say it’s not as bad as Merrill’s or Anders’ from what I know (I think those two are the worst, because they’re the most dependent on Hawke). I remember there being a lot of conflict in it, and actually really not liking Fenris at first because I thought he was always like that. He yelled at me for giving him gifts, and for being nice to him. I didn’t much like it lmao. People have told me his can be played so it’s one of the better ones, but I don’t think I managed it. I just remember being REALLY uncomfortable with all the yelling. I hate anger like that in relationships. Always. Angst is fine, but no anger. Antagonistic relationships are not for me. Keep them over there.
Anyways, I think Anders is pretty confused (and annoyed; we know how he feels about Fenris and the idea of Hawke being with him, especially since it’s implied Anders falls for Hawke regardless) by a pro-mage romance with Fenris most of the time. With a friendmance, he’s probably like ‘how can they get along so well despite disagreeing on mages?’. But with the rivalmance, he probably doesn’t see any reason why they should even be a thing. They don’t agree on anything that matters (in his mind), Fenris just yells at them a lot, they’re not even in a relationship proper till Fenris’ Questioning Beliefs in Act 3 (just basically in limbo), etc. And Anders, who is very close to a pro-mage Hawke (even if you don’t subscribe to the idea that he falls for them anyways), legitimately does not understand what Hawke sees in this guy, someone Anders sees no redeeming traits in.
And I think it does upset him, in a way. Sort of a ‘Hawke deserves better than you, someone who actually understands them’ idea. Whether that’s true or not is irrelevant because that’s how Anders would see it. He wouldn’t understand why they stayed with someone who didn’t seem to like them, or understand them, and it would frustrate him because there were other options, people who did understand and agree with them (potentially such as himself). Because Anders does strike me as the jealous type, even though he probably kicks himself for it every time, not missing the irony given his ‘you deserve better than me’ lines, and getting angry with himself for being ‘selfish’ in that way.
But yeah. Anders’ feelings about Hawke ending up with Fenris aren’t positive to begin with, but are probably worse in regards to a rivalmance, especially one triggered by Hawke being a good person, just pro-mage and wasn’t great at dealing with grumpy elves. It’s a very antagonistic relationship, and I think he wouldn’t like Hawke being involved in that, or getting involved in a relationship that involves near constant fighting just because Hawke likes mages. And this is all from Anders’ perspective, of course, in case that didn’t come through.