I don't know why, but riot has a problem in giving champions the same character they seems to have in stories and teasers.
The best candidates that come to my mind are:
Star Guardian Ahri: In every trailer she's deep, serious. But the in-game voice is totally different. Quotes has like the same character as the base one, she's conceited and vain.
Same thing about sg xayah and rakan: xayah should be really dark and corrupted, but in game she acts as the base one, edgy and arrogant.
Rakan it's more like the base one, but he lacks the dark/sad feel too.