Quick Answer
The best offensive Backup for an Assassin team is Dragomir from the collected material. The best defensive Backup is Grace. Rip is a situational secondary Backup when the team specifically benefits from his Vanguard/Assassin-related support.
Dragomir as Offensive Backup
Dragomir is the clearest offensive Backup candidate because the material connects his B upgrade and Backup value to Assassin-style scaling. Use him when the team already has the survival layer and needs more damage, Shards, or kill-chain pressure. He is not a generic defensive patch.
Dragomir fits best when Assassin, Shard generation, Crit, and kill pressure are already part of the plan. If the board cannot survive, adding offensive Backup may not solve the real problem. In that case, compare Grace first.
Grace as Defensive Backup
Grace is the best defensive or general-purpose Backup in the material. She helps teams that need to survive long enough for Assassin damage, Crit scaling, or Shard engines to matter. This makes her especially useful when your main Assassin comp has enough damage but keeps dying.
Do not treat Grace as only a beginner crutch. The material frames her as a strong Backup piece in defensive contexts. In harder modes, the best Assassin team may be the one that survives, not the one with the highest theoretical damage.
Rip as Situational Backup
Rip is situational. The source material mentions him in Assassin and Vanguard/Rush contexts, but he is not the blanket best answer. Use Rip when his specific tags and effects match the team's trigger engine.
The setup rule is simple: pick Dragomir for offense, Grace for defense, and Rip when the comp has a clear reason. If the reason is not clear, the backup slot is probably being guessed.