[@Beloss]

The skill doesn't rise Atalanta's parameters, it's like [@Raineh Daze] said. A skill meant for countering others after letting them have a short momentary advantage. I really think that it'll not be enough to make out the difference in arm wrestling. Normal wrestling would be more balanced in her favor, since it's already based on countering to redirect your opponent's strength against them.

Either way...

As for Walgrave, the reasoning is simple. He was the one that proposed the deal, making his malice evident first. Of course Atalanta is wary of everyone's there. I even commented on how she knows that this kind of expedition tends to have a bad end for most of those involved in it. The thing is that she's more likely to trust Medea than anyone else, since Atalanta has a debt with the Witch of Colchis and was also a victim of the Argo expedition. That said, she knows that Medea would sell her mom for revenge, as long she doesn't spite Medea it should be safe to be around her.

The magi are a bunch of snakes in Atalanta's view, of course, but she has base to believe that Walgrave is a dangerous one to have around, much more than any other. Considering that Archers are known to do as they think is best, even when told the contrary (unless a command seal is used, of course), it's not OOC for her to kill him if she gets the chance. For an example of an Archer doing similar things, just check how Robin Hood assassinates Hakuno on F/E, despite Dan telling him to not do any underhanded move, but not enforcing it with a Command Seal, on one of Extra's Bad Ends.