Player A targets a unit with Stupefy. Player B reacts with Riposte, targeting Stupefy and the unit Player A targeted. Player A reacts with Defy, countering his own Stupefy. When Riposte resolves, will it give additional might to the unit?

No, Riposte will not give additional might to the unit.

According to the rules of the chain and spell resolution (Rule 356.3.e.7 and 356.3.e.10), if a spell's target becomes invalid or unavailable by the time it attempts to execute, that instruction will not execute.

Sequence of Events:

1. Player A plays Stupefy (Target: Unit).
2. Player B reacts with Riposte (Targets: Stupefy and Unit).
3. Player A reacts with Defy (Target: Stupefy).

Resolution (LIFO - Last In, First Out):

1. Defy resolves: It counters Stupefy. Stupefy is removed from the chain and sent to the trash.
2. Riposte resolves: It attempts to execute its instructions. However, one of its targets (Stupefy) is no longer on the chain.
   • Because the instruction "Counter that spell" cannot be performed (the target is gone), the subsequent part of that instruction—"and give that unit +equal to that spell's Energy cost"—cannot calculate a value.
   • According to Rule 356.3.e.12: "If the spell checks information about a target that is no longer legal... that check returns 'null' and all calculations based on it are ignored."
3. Since the energy cost of a "null" target cannot be determined, the unit receives no might bonus.

Final Result: Stupefy is countered by Defy. Riposte resolves but fails to provide a might buff because its primary target (the spell to be countered) was already removed from the chain.