She was the big boobed clutz, the price he got for saving her in hueco mundo who also happens to resemble his dead mother. Tbh I always found that parallel rather creepy. As Nymroth said, her character type are a dime a dozen, though the same is true for the tsundere type which Rukia falls under (at least in the first arc, in soul society she got humbled quite a bit making her more likable). In classcical (battle) shonen tradition, I'd go as far as saying that the romanic interests are generally tacked on fluff anyway and most of the time the MC's involvment with their future spouses is purely based on one of the females liking the MC, not actual interest from the MC towards them. This is a general theme in DB, OP, Naruto, etc, the usual WSJ fare.