There are few reasons for this pick, so let me explain before you all scoff and start mumbling curses at me.
First of all, Mercy is a support character, meaning she doesn’t attack directly, unless she needs to, and is more focused on keeping her team strong, healthy, and connected. Now, you may be thinking: “this doesn’t sound like Lance at all!” but let’s consider his character for a moment. Lance is well-rounded guy for the most part, dubbed a ‘jack-of-all-trades’ on the Voltron Wiki page, and for good reason. He is able to adapt to his surroundings quite easily, while the rest of the team may become panicked or rash in their decision making (I’m looking at you Kogane.)
Lance thinks of himself of as a fifth-wheel “or seventh if you count Allura and Coran”, but we all know that isn’t true. Lance isn’t a wheel. He’s the goddamn hub! He’s the axle that keeps the wheels connected! He’s the-ok that’s enough wheel metaphors, you get my point.
Lance is what keeps the team together.
Mercy is what keeps the team together. You rarely see a team (a good one anyways) rush onto a point without their healer close by, and if they do, they usually don’t do all that well.
Still need convincing about Lance being the glue of the team and thus Mercy? Sure!
Mercy’s staff is accurate af. She just needs to be within range and her healing/boosting powers are able to hit their target. Sharpshooter much?
Mercy’s ability to revive (her ultimate) can be related (with some imagination) to Lance’s ability to increase team morale, thus ‘reviving’ team spirit. It hasn’t been explored much in the show, but I know Lance’s character well enough to think of him as the more than the comic relief. He knows how to judge a situation (somewhat) and alter his attitude to help the team in said situations, usually through bad jokes, thus lifting team moral and keeping them from growing apart (like I think might happen in season 3, with Shiro missing and all that. Fingers crossed for some Lance development).
This last one is a bit of a stretch but I’m going for it anyways:
Without Lance, Pidge and Hunk would not have rushed down to save Shiro. I mean, Pidge may have, but she would probably have missed Keith rescuing Shiro on account of her short ass legs, and the whole story would have gone to hell (or maybe not but who knows). Lance was the first in the room. He got there before Hunk and Pidge. Why?
He knew that mullet. He didn’t want to lose to it again. Keith is/was his rival, and there was no way in hell he was going to be second to that guy again.
(Also his legs are long af that boy probably Naruto ran down there.)
This could be related to Mecy’s ability to “angel glide” towards members of her team, knowing when they are in danger etc etc.
You could also say that Lance has the ability to sense when others are in danger. We’ve seen it several times throughout the show, and this is similar to Mercy’s ability to see which characters need healing on her team.
There’s also the fact that Mercy is usually always targeted first (since she’s healing and what not), and have we not seen Lance get into a number of perilous situations when his team isn’t there to help?
The healing pod incident, the air lock, the bomb, the being-tied-up-by-pretty-ladies, you get my point.
Without their team for support, both characters sort of breakdown. Without the hub, the wheel won’t turn, and without wheels, what’s the point of the axle (do these even make sense???)
I’m sure many of you will still disagree with my choice, especially as Mercy, so let me try one last tactic:
Lance with wings.
Like…can you not see him rocking Mercy’s Valkyrie outfit? And don’t even get me started on the catch phrase “Heroes never die”. It’s got Lance written all over it.