Basically, an ex-minion who's dedicated it's life to the person (or group) who rescued it / didn't kill it after defeating it's old master.
PFS, with no special boons to start, so I want to go goblin or kobold. (Could go other, but that just feels right.) Was leaning necromancer apprentice as background.
First, that is a really cool character concept.
Necromancer Apprentice (Book of the Dead) would certainly fit. I could also see Cultist from Advanced Player's Guide.
Amulet Thaumaturge from Dark Archive would certainly get you that Champion's defensive feel without any required religion flavor added in.
But the character concept could be done with any of a bunch of other classes too. Probably a martial class of some variety, but Fighter, Monk, or Rogue from the CRB could do that, as could Inventor from Guns and Gears.
Looking for other "bodyguard" options I'm missing
For defending others, I think the Fighter does it better than most.
They can get the Shield Warden feat along with Champion, and can also get Guardian's Deflection (though a couple levels later than Swashbuckler does). Fighter and Monk and Gymnast Swashbuckler can all also be very well built into grapple/trip/shove control fighters that do well at locking down an enemy's ability to move and attack other allies.
Alchemist could work with the concept but give a very different feel — especially if you went for Mutagenist as the research field.
You were the guy that mixed up all those chemicals for the boss. Provided various elixirs and mutagens that they would then use. The ‘bodyguard’ aspect would be via defensive elixirs rather than stepping in the way of an oncoming attack.