auxiliary intervention
Legal term: a third party joins an existing lawsuit to support one side, without becoming a full party. Used in civil procedure contexts.
A company appeared to intervene in support of the defendant.
The requirements for auxiliary intervention are stipulated in the Code of Civil Procedure.
訴訟参加 is a broader term for intervention in a lawsuit, while 補助参加 specifically refers to auxiliary intervention to support an existing party.
auxiliary intervenor
Compound of 補助 (ほじょ, 'assistance') and 参加 (さんか, 'participation'). A straightforward legal term formed from its constituent parts.