Belaira and Marshtomp are very close.

Not As Friends.
Not as Mates.
As Siblings.

She isn't related to him but they share the same bond as a Young Brother and an Older Sister. She does bully him a little but she still loves him and would do anything to keep him any serious danger.

Belaira also trains Marshtomp.
Between
Learning Special Attacks from Mrs Marine
and Physical Attacks from Belaira,
He might turn out be a very strong individual