And also whether they each do something that’s unique only to them. (Off-topic, the shade has to be my favorite familiar compared to the rest, with the spider being my second favorite.)
AchtungNight’s Lost Heir guide has one for all the familiars.
Here are the places you can get them:
You can get familiars at the start of LH2 (different ones for the 5 possible starting areas) if you choose to be a level 2 Druid, Ranger, or Wizard.
You get the Shade familiar if you study the Book of the Dead and become a Necromancer in LH2.
In LH3 you can make the Monkey your familiar if you have a class level in Wizard, Druid, or Ranger (you do not have to be Level 2 of those classes to get a familiar in LH3) and lack a familiar already.
If you get the Demon Master class (and have no familiars, I think) in LH3 you get an Imp familiar.
Different familiars have different combat capabilities. My personal favorite is the Firedrake. It is a good all-rounder and goes particularly well with the Dragon Rider class.
Sadly, you can only get one Familiar in the LH Trilogy, however there is a case where the MC obtained 3-4 Familiars (If Dragon counts as one) in The Last Wizard.
Yes yes it does