1. Axis...do I really have to go into how amazing her passive is?
2. Emma is hard to start with--since she doesn't start with any cash, her passive doesn't mean much for the first few levels. However, once she gets going, she can min/max better than any other pilot.
3. Meg's passive could use a better explanation. Then again, I haven't spent enough time trying to learn the item level mechanics.
4. I'm not a huge fan of Tuck, since I like to explore the levels myself, but I don't think he's underpowered. One thing: bots should not be able to take advantage of his scouting in Hostage Rescue missions. Having every single bot run towards a pod is very annoying (as you can't really chase them down).
5. Genji is a huge early game boost. As that is when the player is at his weakest, he is pretty good for Expert. That is kind of lost if you play on lower difficulties, but he is still a good pick.
6. I haven't tried Rey yet, so I cannot comment too much on him. Based on some of the discussions, and my reasons for not taking him, how about the following: allow him to get extra loot if his 'extra' exo survives? That would allow him to benefit from every mission, like the other commanders.
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