Tauriel Was a Fine Addition to The Hobbit (Until Kili Ruined It All)
One of the main complaints about The Hobbit movie trilogy was the complaint about addition of new characters.
These characters never existed in the book, never became more than tangentially involved in the central story of Bilbo, the dwarves and Gandalf, and were accused of existing solely to pad the trilogy's length, so that the studio can put three movies into theatres, instead of two.
And of those characters probably the second most disliked was Tauriel (Evangeline Lilly) – but she only could not compete for the first place because Alfrid (Ryan Gage), a total waste of both skin and screentime, existed.
Besides being invented for the movie, Tauriel checked a lot of boxes on the "bad fanfic character" list, from having attitude to having better ideas than every other elf around, to being all-around superior to the book character whose very minor role in the story of Bilbo she took over (though the last part was not hard, as he was a nameless extra).
But most importantly, she was the third point of a love triangle involving two canonical characters (one of those characters being an established sex symbol of the movies' Middle-earth, too).
However, some fans believe than by herself, without the dumb love triangle, Tauriel was a fine addition to the movies. Quite a lot of fans, actually, judging by thousands and thousands of upvotes which this Reddit opinion got (despite being branded an "unpopular opinion").
Well, it is hard to blame them. Yes, the trilogy certainly was overbloated, but a female equivalent of Legolas doing cool things is not something that most people would begrudge a bit of screentime.
Unfortunately for Tauriel and the audience, the love triangle did exist.
There is a persistent rumor among the fans that when Evangeline was offered the role, she agreed, with a half-joking condition that there would be no love triangle (after playing a part of one in Lost). And originally there was indeed no love triangle in the script.
But as we know, The Hobbit movies had a troubled development, with a lot of rewrites, and eventually the love triangle was forced in by the producers. Well, it is hard to say whether the rumors are wholly true, but its presence certainly does feel forced!