But Tahm never names those individuals does he. Kat's father could just as well be who he's talking to Kat about we won't know until RIOT places that story into lore.
The entire "love" affair between Kat and Garen is nothing more than fan assumptions.
It even states on Garens page:
http://gameinfo.na.leagueoflegends.com/en/game-info/champions/garen/#champion-lore
"The plausibility of these rumors was bolstered when, in every instance thereafter, Garen seized the opportunity to encounter the Sinister Blade again. A paragon of Demacian ethic, Garen never entertained such allegations, for he knew others couldn't understand. Even simply the pursuit of a worthy opponent on the battlefield is, to a true warrior, the reason to rise each morning. "
On Kat's Page?
http://gameinfo.na.leagueoflegends.com/en/game-info/champions/katarina/#champion-lore
Not a single mention about Garen other than he is a rival just like she is to him.
Again its an issue of drawing conclusions based upon the perceptions (or teasing) of "other champs" (just like Blitz's dating service) instead of from the characters themselves.
I can tell the world (and tease you) that you have the hots for my ugly brother all day everyday but that does make it true.
[slayer-jinx-catface]