noun
auxiliary intervenor
Legal term for a third party who participates in a lawsuit to support one side, without being a direct plaintiff or defendant.
補助参加人は、当事者の一方を勝たせるために訴訟に加わる。
An auxiliary intervenor joins a lawsuit to help one of the parties win.
Compound of 補助 (assistance) + 参加 (participation) + 人 (person), literally 'assistance participation person'. The exact historical derivation is uncertain; the term is a modern legal coinage.