Jealousy
Friendship isn't the only invisible tie that binds them together. Robin, Cordellia, Severia, Morgan, and even Chrom are connected by less noble ties as well.
I do not own Fire Emblem.
Green eyes make every other pasture seem more fair. Morgan isn't one to hold a grudge, but seeing the way Severa treats their parents is enough to spark one.
On one level Morgan knows that's just how her sister is: Severa is caustic because she's scared and scarred and afraid to lose them again, and even if she can't remember the future she can tell Severa was hurt by the loss of the parents. She can imagine, because she actually can imagine how she would react if Father passed away now. It may have been a kindness that she can't remember Father's death.
But at the same time, Morgan wants to do nothing else as much as grab her sister by the shoulders and give her a nice, hard shake while yelling 'Appreciate what you have while you can, you fool!'. Severa may have hard memories of their family, but at least she has memories at all. Morgan doesn't even have that.
Amnesia might make a bonding topic with Father, she supposes, but it's really not worth it. Not when she can't even share those memories with him, when he hasn't made those memories yet. Not when she has to forget Mother, who only becomes more amazing and incredible the more she sees of her.
Not when she had to see the flinch of pain in Severa's eyes when the normally caustic girl reunited with the sister thought lost, only for Morgan to ask who she was and when she had ever had a sister. Her sister may be the source of her ire at times, but she would never willingly cast her aside.
Morgan is desperately reconnecting to a family she never had, or who had never had her, and Severa acts like it's something worth scoffing at and dismissing. She dismisses the one thing Morgan is craving above all else, those memories of family.
Morgan would give almost anything: her looks, her books, even her tactician's mind, to have the memories that Severa takes for granted.
Author Note: Fourth of five jealousy-themed character pieces, centered around the members of a Main Character-Cordellia family. Here's to Morgan, who never seems to have a bad bone in her/his body (despite, you know, being fanatically devoted to the Avatar of Grima). A little sibling resentment seems normal, though, even to her.