Actually, they arent...
Teemo and Tristana are just friends...
However, she is the only friend of Captain Teemo... so, The yordles tend to think that they are in a romantic relationship, butTristana has said that he is only a friend and, as we can hear in her quotes, only talks with him as a friend and nothing more.
"Um, Teemo, are these mushrooms your rations or your ammo?" - A normal joke between friends.
Some people says that Teemo feels something for her... but Omega Squad Teemo do not talk in a "romantic" way to Tristana in his only quote towards her.
"Rockets ain't for jumping. Who gave you that?" - He almost threat her as an stranger... mainly because Omega Squad Teemo is super serious.
The only yordle champ that actually feels something for Tristana, is Rumble. He called his robot "Tristy" in honor of Tristana and is heavily implied that he has a crush on her... that is mentioned in the JoJ and quotes of Tristana and Super Galaxy Rumble.
"Nice suit, Rumble! What d'you call it?" - This quote implies that she knows the name of his robot, but Rumble has never tell her the name in person.
"Eyes on the enemy, Rumble." - Then again, She knows that he feels something for her... But Rumble has never confessed her love to her.
"How d'ya like me now, Tristana?" - Now that he is the Mighty Rumble Too Much, he has the guts to tell her,
"I'll always come back for you, Tristana." - Pretty clear that he loves her.
"Let's make fireworks, baby." - no comment.
Bonus Plotwist: Tristana actually loves
... just hear her quotes towards him!
"Aw, Veigar! You're cute when you're angry."
"Oh, Veigar! Tell me all about your diabolical plan."