in the start of berserk caska is griffith's 1# fan girl, most loyal supporter, and most powerful and useful follower/fighter aside from griffith himself and later on guts
she is also (as a result of him giving her more attention then the rest of the hawks) the closest one of his followers and the one he trusts most (again until guts is made a part of the hawks)
but while she may have a hero crash on griffith she never actually tries to act on it and ends up being his battle batler insted
once guts is introduced caska starts out being somewhat hostile to him since he seems to gather more attention and trust then she does
kallen starts out (or rather becomes) zero's most loyal follower, most powerfull tool and 1# fangirl as a result of him giving her more attention and trusting her more then the other OOBK
and while she may have a hero crash on zero she never actually tries to act on it and ends up being his battle batler instead
once C.C is introduced kallen starts displaying hostility to her (but not even remotly the same level as caska) becouse she seems to take the attention and trust that was once kallens and takes her place at zero's side
caska keeps up being hostile to guts until ep 12 when they end up alone together and in hiding from their enemies during which time she tells guts more about griffith's and her own story
the two start bonding rather then being hostile to one another from that point on once they are reunited with the hawks and griffith (guts even teasing her about griffith at times, telling her that he's not that far away)
part of the outcome is that caska starts to see griffith as more of a man then an idol that she worships
kallen and C.C end up spending a year together after lelouch is captured and its during that time that they bond (like sisters basiclly) while talking about lelouch
it is at this point that the role of guts for the most part replaces C.C with lelouch himself (zero=griffith)
and while the two characters are very differnt from one another they also share many similarities
both are loners (even when surrounded by other people)
both incredibly talented in what they do (to the point of being a legand in their on livetimes)
and both show zero romantic interest in anyone at all
kallen even tells lelouch that she chooses to view him and zero as different people in ep 2
so for now on its
lelouch=guts
zero=griffith
caska ends up once again separated from griffith when he is captured but at the same time is also separated from guts who left them
the scene where guts leaves is the moment(as caska admits) when caska understands that she has conflict against her feelings
her sense of admiration for griffith became less importent then her feelings for guts (even if she doesnt admit to herself)
she cant get him to stay with her however since he has something he feels he must do
but she doesnt go with him or anything becouse she refuses to admit it becouse it would make everythings else that she has lived for nothing more then a lie
so she sticks around with the hawks and after griffith is gone carries on to fight for his dream (even if the man himself is gone)
kallen ends up being separated from zero and the OOBK for a while and its at this point where she starts taking more interest in learing about lelouch the man rather the zero the symbol
once she returned to the OOBK she ended up watching as lelouch leaves and starts to feel confused about her feelings
she may have started to understand that what she feels is more then just loyalty to zero (even if she still didnt understand that its genuine love towords lelouch) and realizes that she needs to know what she really means to him
but she stays with the OOBK and fights on for all the things they have always fought for (her brother's dream, which ougi told her zero embodies)
from this point on i would also like to address the complaint that many (non-kalulu) fans point out that lelouch hasnt shown that much romantic interst in kallen (or indeed anyone)
guts had not (till that point) expressed any feelings (romantic) at caska or anyone else for that matter and said that he views her as a brother in arms rather then a woman
but once they hooked up it made perfect sense that a man like guts who doesnt show any emotions or needs for love is exactly the kind of man who NEEDS it
caska and guts meet once more and she is hostile to him becouse she says that he was the reason that griffith had fallen and partly becouse he betrayed them
but after some prompting she starts to talk about how her own sense of what does and doesnt matter has changed and she cant keep living just to fight for another mans dream
its only after he left that she had to watch him walk away from her that she understood that she really loved him (which didnt stop her from attacking him at first)
she and guts end up becoming lovers from that point on (till the ending
)
kallen and lelouch meet once more in ashford and despite her attempts at getting him to respond to her (to stay with her) he has his own thing to do and so they part ways after the kiss (which the VA said was the moment she understood that she loves him) kallen stays with the OOBK and when he "betrays" them she is ready to attack him