Details for how to recruit - or more accurately, how to retain the services of - Kaiden Alenko.
Recruiting Kaiden is easy, you don’t even have a choice in the matter. He’ll be assigned to your squad during your first mission on Eden Prime and he’ll remain a core component of your team until you manage to recruit another character on [the Citadel]. Only then can you bench him in favor of another character, should you so desire.
While recruiting Kaiden isn’t much of an issue, keeping him around for the long haul is a bit more complicated. During the mission on [Virmire], when you have to choose between relieving Ashley or Kaiden, you must choose to help Kaiden if you want to keep him around. You can find out more about this choice by following the link - beware of SPOILERS.
You’ll unlock the Sentinel Ally achievement for completing five missions and/or assignment with Kaiden in your active party.
ME1: Complete 5 missions with the Alliance sentinel squad member