I'd like Aramis (with his uncanny marksmanship) as a sentinel, with Porthos as his guide. It can be established, or something happens to trigger Aramis and Porthos just acts instinctively.

Could be set in a universe where sentinels and guides are known. Or alternatively, maybe they have no idea what's happening, and they work things out through trial and error.

Perhaps also feature Athos and d'Artagnan, as sentinel and guide.

Slash or gen is fine.