"Let's assume both Mew and Celebi (who is female) is in love with Mewtwo. Mewtwo is not indifferent to either of them. Now, when Mew was about to reveal her feelings to Mewtwo, Celebi does so before her. How would Mew react in this situation according to you?"
Possibilities here...
1-Mew reveals her feelings just a few seconds after Celebi revealed her own feelings and Mewtwo chooses. If Mew is chosen, she is happy... else, know that there is no problem Guillotine cannot solve!
"What?!? Celebi!!!" screamed Mewtwo. Mew whistles innocently... and then turn to Mewtwo, smiling in a naughty way. "What? Why are you looking at me like this? It's not my fault if her head fell off!
2-Mew look down depressed and go drink a good bottle of milk to forget... or if she's being stupid, liquified energy crystal, getting herself drunk. She may also go seek that brainless Lucario to forget through a tiny "physical workout"... lucky Lucario!
3-Mew starts to cry loudly, upsetting Mewtwo and getting his attention. As Mewtwo take her in his arms and try to get her to feel better, Mew taunts Celebi, who is fulminating. Mew: 1. Celebi: 0.
4-Mew look down and fake out leaving the room... but actually decide to beat Celebi at her own game! She flies toward Mewtwo at maximum speed and kiss him while telling him with all of her heart that she loves him through telepathy. This will make Celebi upset, of course... and cause her to protest.
Now, several possibilities on how this case ends...
-Mewtwo chooses Mew. Celebi is jealous and attempts to kill Mew... but fails.
-Mewtwo chooses Celebi, finding Mew's declaration a bit too sudden and shocking. Mew cries loudly.
-Mewtwo chooses both. Yay for triple relationships!... err... weirdness. Ignore this option.
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Well, that's my own theories for my own topic!... now to the topic of someone else!
"After being together for so many years, Mewtwo ends up liking another female and dumps Mew for her. What'll be Mew's reaction and what'll she do?"
Well, it vary on if the female is mortal or Legendary.
In the first case, Mew will first be shocked and ask Mewtwo why this happened. Learning of his feelings, she will warn him that following hormones may not be the best thing to do and that he may get hurt when he fall out of love with that new female or when that female dies.
Now, Mewtwo will either listen and return to Mew, bearing with the emotionnal pain until it disappear, Mew trying her best to help him endure his own hormones... or Mewtwo leaves her anyway.
In this case, Mewtwo will probably be happy while Mew will be really depressed... but she will keep her optimistic point of view: Mewtwo will eventually return to her. After Mewtwo falls out of love, he will realize who was the one he was supposed to be with and will return to Mew.
If he doesn't fall out of love and that the new female dies, Mew will help him forget and remain with him as a friend while he recover... and once he has recovered, Mew will wait for him to make the first move. Again, two theories: he return to Mew and the two are once again happy... or he goes for another female, again. In the second case, Mew gets annoyed and warn him that he went through this already. If Mewtwo listen, the two are happy together and stuff... else, Mew's patience reaches it's limits and she pounce on Mewtwo's face, turning his head into raw hamburger in seconds with her claws (Hey! She's a cat you know!).
Now, that's all for the "female is mortal" case... now for the "female is a Legendary case"...
Mew will not take it kindly. She will accuse the Legendary of having brainwashed her boyfriend/lover and will be extremely jealous... although in a non-violent, non-threatening way. She will become very cold toward that Legendary if it once was her friend and try to remind Mewtwo of what they did to get together and of how their love was stronger than this.
Possibilities here, again. Either the Legendary's love is hormonal, either the new love is stronger than Mew's was. In the second case, Mew will be depressed and get drunk on liquified energy crystal to forget... that or she will take Lucario as a substitute in hopes of getting Mewtwo jealous. If it works, she dumps Lucario on the spot and get together with Mewtwo again. Else... well, back to her old way of life.
As for the first case (Mew's love was stronger), she will try to remind Mewtwo of their time together and will give him proofs of their stronger bond. If Mewtwo is not blinded by hormones, he'll fall out of love progressively and come back to Mew... else, Mew will either wait for Mewtwo to fall out of love or, if the Legendary is manipulating Mewtwo to force him to remain in his hormone-driven state, she will bring out the big canons and try to beat up the Legendary. Since Mew is, well, Mew, she WILL succeed... but that brings two possibilities.
First, Mewtwo realize that he loved Mew. He may or may not dislike her for a certain period of time before realizing it but he'll get together with Mew again. However, it might be possible that he will not forgive Mew and ignore his heart. In this case, the two kitties will get in a feud with Mew who will keep on fighting until she figures out that it is a lost cause and then, she will give up, depressed... that or Mewtwo realizes the truth, again.
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Yes, that was long... but I hope it was interesting and not too confusing! Have a nice day!