Someone in an Asterix forums recently complained that Obelix never got a proper love interest, and it got me thinking... I mean, it seems that across Asterix, Lucky Luke, Tintin and probably many other popular series, very few (or outright none?) of the main characters discovered a bona fide romantic partner, and I suspect that the difficulties of mixing romance with a humor format was one of the biggest reasons why.
That, and the fact that adding a love interest would generally mean that such a character would become part of the ensemble, which means another mouth to feed, so to speak. (more storyline & panels for them, etc)
This is kinda why I'm re-reading Henk Kuijpers' Franka series, tomes 14 & 15, in which Franka has a pretty exhilarating love / adversarial relationship with "Rix," an art thief she initially sets out to capture. It's sort of in the style of James Bond films like From Russia with Love and The Spy Who Loved Me, and I thought author Kuijpers brought it with a lot of style and interest:
Now, I suppose that the difference in Franka (compared to more directly humorous series) is that such a series only lightly relies on humor, and maybe has greater license to muddy the waters without getting bogged down. For example, "Rix" could easily have been killed off either immediately or down the road, with the spirit of the series suffering little or no detriment. (much like a Van Hamme series for example, such as Largo Winch and Lady S.)
Compare that to Asterix, Lucky Luke or Tintin, in which it would have been a notably tragic event, doubtlessly shifting the tenor of the series. For Asterix in particular it could have been plainly disastrous, offending readers along the lines of how Simpsons viewers were outraged by the episode which revealed that Principal Skinner was in fact a fraudster.
All that said-- I'm hardly some 'know-it-all BD/Euro person.' So maybe in some other series, particularly humorous ones, romance can work perfectly well..?
Regarding your last question, there is Valerian and Laureline. https://womenwriteaboutcomics.com/2017/12/the-best-thing-i-didnt-know-about-valerian-laureline/
Cool, thanks for that link! It seems there's a point being made there that somewhat echoes one of my own, in that while the two leads are affectionate, the albums themselves don't seem to devote too much time & space (haha) to them being romantic or outright sleeping together. That seems to reinforce the idea that such things might be at odds with the overall aim & philosophy of the series. *shrug*
In any case, Laureline was conceived from the beginning as a main character, right? So what I'm curious about is when a successful series later adds a main character via the mechanism of a love interest.
Smurfette from Les Schtroumpfs is sort of a sideways example, but in her case she was moreso just a main character being added that wasn't the GF of anyone in particular.
Or if you remove the romance angle, then the biggest examples I can think of right now would be the Tintin series adding Haddock and then Calculus. Altho they obviously worked out beautifully, there was always the chance that readers wouldn't appreciate them, requiring Hergé to 'fade them out' one way or another.
I feel like there's got to be an example when things like that don't work out, but I think it also goes to show why so many series have been cautious about that kind of thing.
EDIT: Oh wow, I just double-checked and realised that Laureline was not originally a colleague of Valérian, but rather a peasant girl from 11th-century France who joined up with him in the debut album Bad Dreams. So she technically was added to the series, but since we're talking the very first album... hmmm. :S
EDIT2: Ooh, I just thought of another candidate! That would be Nävis from Sillage (Wake in English) hooking up with a Human-like male and producing a son ("Yanno") who later joins her in the adventures.