While the following technically has spoilers, the explanation isn't told.

Morgan, Chrom noted, seemed more like his mother than himself. He believed it was the whole amnesia thing (was Robin's side of her family cursed with memory loss?), but there were times in battle Morgan held his sword a certain way that was more similar to the technique he possessed. Perhaps it was just him, or that while the lad couldn't remember anything about him, his body did.

The idea of sparing with him seemed like a good one. Of course, Chrom asked Robin if it was alright; she threaten him of a good thunder spell should he harm their son. Practice swords were the replacements as a result- he wasn't in the mood to get electrified.

Before the event was to come, Morgan kept on thinking over the arrangement when Robin approached him, holding the infant version of his older sister in her arms.

"Morgan? Is something wrong?" she asked him. "You're pacing around."

"Huh? Oh, nothing, really. Lucina told me the last time I spared with a family member, it was with her a year before coming to this time," he explained. "It does kinda feel that way- it's vague, but all I remember is helping her protect an area in Ylisse."

(More to come.)