is recruiting can be part of a chain?

ex: it's my recruit phase

opponent play a plot, can i recruit a carac as a response and then play the carac ability, before the plot resolves?

you can only recruit characters and equipment on an empty chain, so you cannot respond to anything by recruiting a character.

mm and if opponent have mephisto5
and i recruit mephisto 5 can my opponent respons and empty the ko pile befor my mephisto hit the table?

yes... he can do that...