If my opponent attacks and puts a 'When I attack' trigger on the chain, when I reveal Mischievous Marai from Hidden as a Reaction, does it stack directly on top of the opponent's existing chain or start a new separate chain?

Yes — it stacks directly on top of the existing chain. Riftbound allows only one Chain at a time, and any card or ability played while a Chain exists is placed on that same Chain (Rule 330.1–330.2).

Mischievous Marai's Hidden status grants her Reaction, so she's legally playable in the Closed State during your opponent's attack-trigger chain. She's added as a new Pending Item on top — she does not create a new chain — and resolves before the opponent's "When I attack" trigger (LIFO).

One extra wrinkle: once Marai finalizes and enters play at the battlefield where she was hidden, her "When you play me to a battlefield, deal 2 to an enemy unit here" also triggers and goes on that same chain, on top of everything else. Reactions played mid-chain always join the existing chain (FAQ #2952).