Mechanically, the Bond stat is raised by 1 on a Pokemon for winning a battle in which it participated. All Pokemon in the party (and the current fusion, if applicable) gain 1 Bond for every valid tick of RP. In either case, a Pokemon of the trainer's favorite type will gain an additional point of Bond whenever a point is granted. Being fused gives an additional bonus of 1 point on the fusion.
Fusions have a 10% boost to Attack, Defense, Special Attack, Special Defense, and Speed over the Pokemon's original form.
The level of a fusion is dependent on Fighter XP (FXP). This is effectively the trainer's experience as a Pokemon, and is raised in battle the same way. The calculations used to determine the Fighter XP of a fusion are as follows:
Temporary fusions use the lower total of the trainer's TXP or the Pokemon's XP.
EXAMPLE: Joe (10,000 TXP) tempfuses with his Arbok (30,000 XP). The Arbok-fusion has 10,000 Fighter XP, and is level 21.
Permanent fusions add 1/3 of the Pokemon's XP as Fighting XP at the time of fusion. The Fighting XP total is used for all permanent fusions.
EXAMPLE 1: Joe (TXP irrelevant) permfuses with his Arbok (30,000 XP). 30,000 / 3 = 10,000. Joe gets 10,000 FXP added and is level 21.
EXAMPLE 2: Joe then permfuses with his Clefairy (20,000 XP). 20,000 / 3 = 6,666. Joe gets 6,666 FXP added for a total of 16,666. He is a level 25 Arbok-fusion and a level 27 Clefairy-fusion.