Elle was at first prejudiced against him, being that she was jealous of Kinji for earning the respect and feelings of Aria. As such, they were at an emotional impasse against each other. They appeared not to get along during the time period Elle was keeping her secret from Kinji.
However, after Kinji saved her life and treated her femininely (under the influence of Hysteria Mode, of course), Watson had obviously developed an interesting amount of feelings for Kinji. Such that, Watson would even allow Kinji to perform intimate acts with her. Merely choosing him shows how much respect and emotion Watson potentially harbors for Kinji.
Watson was formerly Aria's fiance, although it was arranged marriage decided by Aria's grandmother. However, they had never met prior to the incidents of Volume 8. Aria even confessed that she was doubtful of Watson's mere existence.
While Watson used a suave and smooth tone in attempts to win over Aria similarly to how Kinji would act in Hysteria Mode, it is unknown if Elle's masculine upbringing extended to Watson actually having some sort of feelings for Aria above the boundaries of friendship.
Perhaps its due to both of them not knowing who each other's respective ancestors are, but neither of them seem to particularly like nor dislike each other. They can be considered to just have a mutual friendship.