you will need to create an 'IK rig' for mr Brekel, then a 'IK retargeter' choosing his skeleton both times upon creation.
then you can proceed to retarget to the unreal mannequin (recommended) or your custom character with its own set up ik rig.
best way to understand how ik rigs work is to import 3rd or topdown starter content and study the mannys ik rigs
hope this helps.