Gihan — Today at 7:57 AM … Does an always on trigger-passive like Rena precede sequencing of Reactions, so Rena can intervene before Henri can trigger his reaction? We know that instead of last in first out stack (MTG) or alternating window, the initiative player will choose sequencing of windows to react. We assumed the “cannot is final” rule that exists in card games (couldn’t find it in the big rules). But by making Rena a passive, you open up a lot of FAQ issues. This is true in a lot cards with current wording . In fact, many games used “Forced Reaction:” (occurring before reactions, sequenced in the initiative player’s order) because passives with a trigger (not card changes) are a nightmare in the long run. Unless you want a FAQ for such cards with interactions, updated with new releases, it is far cleaner to devise a robust lexicological template that minimises such issues (maybe compromise design slightly if it causes issues). This was why Nathan French released AGOT 2nd Ed (remember 1st Ed’s monstrous FAQ), why Rosewater removed “Interrupt” and introduced the stack to MTG.
Dragonhatter — Today at 10:00 AM Henri reaction is when he starts the challenge action, Rena is when there is an opportunity to intervene which doesn’t happen until after Henri would have has a chance to use he reaction and resolved it.
CroyLCM — Today at 10:01 AM Rena says you may, Henri is a reaction you may not activate. Even if both reactions, Neither would be forced. Their timing is completely different.
Agent006 — Today at 10:02 AM OP mentioned some vagueness, as if Rena could bypass the cannot,
Agent006 — Today at 10:04 AM No, i think it was the bit of could Rena intervene even after Henri used his reaction.
CroyLCM — Today at 10:05 AM No. If henri activates and prevents intervening, then Rena cannot intervene. Even while engaged 
_Answered in Discord:_Discord