507.1C Any player may play a reinforcement game-based effect that reads “Target defender you control has reinforcement this attack,” with a cost of exhausting a support row character that shares an affiliation with and is adjacent to the defender. (See rule 708.7b.)
701.10a Characters attacking a character with reinforcement can't cause breakthrough.
These entries from the complete rules document should answer your questions. Only defenders can be reinforced during an attack. Reinforcement only prevents breakthrough. Reinforced characters still take stun damage. Defenders do not cause breakthrough, so reinforcing an attacker will not prevent any damage. A character needs "invulnerability" to prevent stun damage.
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