ZigzagPX4
The Swiftness of the Ranger
They thankfully played it safe with Kellogg, and referred to the major factions already known like the NCR and Shi without adding anything new, probably due to not wanting to interfere with the lore on the west coast.
Kellogg felt overrated to me in how so many people consider him well-written. It's like he's a fanfiction OC for Fallout 2 and New Vegas. Especially how his history is introduced - vague, not detailed, but sure to mention all the "cool" parts like being from the NCR. And then the scene where he was meeting with the Institute, how he's completely relaxed in taking out three Institute synths. (The Institute woman's dialogue was the worst part of that entire quest.)
Short of the Sole Survivor beating him, he was basically a Mary Sue. Badass mercenary with tons of experience, several skills, can handle himself anywhere, was born in the NCR and worked for the Shi. The abusive father and dead family cliché to top it all off. Then he beats up synths right in front of the Institute, gets himself hired by the Institute, becomes seemingly immortal (I know he isn't really but that seems like the tone they want to set), then the way Nick Valentine describes him as one of the best, most skilled mercs in the wasteland with the incredible skill to pull off a high-risk kidnapping mission.
He ends up being associated to every Fallout game and if he wasn't beaten by us then he would've basically been an "invincible, immortal mercenary with a dark past and ties to every faction in the wasteland". If that's not Mary Sue levels of wish-fulfillment fantasy and flawless character, I don't know what is.
Kellogg was, to me, a self-insert "badass" character made by either one or a bunch of devs trying to throw what they think are unique ideas and their own "awesome" character in. It amounts to levels of writing that I've never seen in anything above poorly-written fanfiction.