I think I'd be fine with Mari and James being with most anyone, really. Nobody that's really waving their hands and yelling "HEY, PICK ME!" to me.
*Looks at top five*
*Sees Mari in first place for Kagura's partner, James in second, potentially third place for Sean*
I NEVER LOVED YOU ANYWAY!!!
But seriously, would you like to pair up? I see Kagura and Mari getting along because I got the vibe that Mari would be willing to make the partnership more of a friendship once she is assured that her partner doesn't see her a living battery, which definitely isn't Kagura's view on the partnership.
James, while lacking a defined goal, is definitely driven, and is therefore someone Sean could throw his support behind wothout worry. Though, to be honest, I'd rather Sean go with a different Tuner if you agree to the Kagura and Mari partnership, if only so that BOTH of our characters don't end up stuck in a rut if one of us is busy one day, which I have seen and experienced all too many times in Fate RPs.