How can I choose the character that other players attack me with?
Its based on the last time you fought with as long as theres not a big drop in character level (IE using a champed Iceman and then swapping him for a 2*, Iceman would still be on your defensive team)
You can’t choose the characters that other players attack you with, per se. You can, however, use the skip button to filter through teams and decide which one’s are more favorable for you to attack.
Obligatory mod-voice: You can also find great resources in the Tips and Guides section of the forum