In Co-op mode, Mei doesn't change at all, as her recovery stat does not rise nor is her hyper price or effect (with the exception that it cannot harm friends).
When holding, attacker can be one of Mei's better roles, as her increased defense and evasion make a nice accompaniement to a +3 ATK. Unfortunately, however, her viability is hindered by her low HP and potential to lose her +1 stats, thus it will likely be beneficial for another player to play as guardian or supporter to help keep Mei in good combat-ready condition.
Similar to attacker, within hand, mei can be a decent guardian, sporting a +3 DEF, which will enable her to survive hard blows from the boss. She is not a great choice, though, as her HP is low, providing few shields, and if she is KO'd, she will lose her +1 stats. The player would need a supporter ally to be around at all times, which is problematic as the attackers and avengers will likely need their assistance more.
Supporter is not a great matchup for Mei, as she doesn't have any heavily increased stats on her own, her passive gift gives limited protection and will drop if she is KO'd. While the role does provide a +1 evasion boost, it will not increase the her survivability much during the late game, and the player will suffer from Mei's lack of damage output.
Given her low HP pool, Mei is not an ideal candidate for an avenger. Even when holding, Mei will not be able to take much damage, and as a KO will result in her losing the card, it will likely not pay off for the player. While the role does provide a +1 evasion boost, it will not increase Mei's survivability much during the late game, and her lack of damage output will more than likely hinder the player's progress.
As noted by her rank, one of Mei's best roles is as a dealer. While a bit hard to accomplish, this mainly stems from the ability to giveto other players, which can be particularly potent on attackers. Some notable combos for this are attacker who can wield a permanent +5 ATK and +3 EVD thanks to her passive, , who once powered up with Active Extensions can have a whopping +5 ATK +4 DEF and +4 EVD, and attacker who can have a frightening +6 ATK. Otherwise, it can pair decently with most other characters, however, there are exceptions. The card can be problematic for characters like who need a negative defense, and avengers, as they need to take more damage, and thus a defense boost may pose a problem.