Short version: Pure luck and dice that love him.
Very long version:
Raggi Ragnarsson was essentially a throwaway NPC when he first came along. I made a pretty tough berserker based on the Barbarian template from GURPS Dungeon Fantasy 1.
You see, there is a room in the Caves of Chaos that houses a 4th level fighter the PCs can free. Raggi took the place of that guy.
He was a prisoner of gnolls, and an unapologetic bandit raider/reaver/adventurer type. He was enslaved along with some of the people he'd been raiding and had not a twinge of regret for his actions.
I figured, he's an unarmored berserker, he'll either die in combat or the players will kick him to the curb after a session or two.
In his first session, Raggi was free and cheerfully hacked up a bunch of gnoll kids. The players cringed - it was pretty harsh, and worse it was a bad idea (do you want to negotiate with guys who kill your kids?) But next time the players needed another fighter, he was around, so, okay, bring him.
He charged into a mob of orcs and got hacked up. Oh, but he killed, crippled, or knocked out everyone who came close. He even killed the orc chief before dropping. The players assumed he was dead, but within minutes he got up and started talking. The players basically decided to let him get killed. Yeah, come with us, they'd say, and then let him run into danger and hope he'd get dead.
He'd get beat on, but just wouldn't die.
Since then he's been a serious help to the group. He's tough to control (he tends to go berserk and charge into bad situations), he's dangerous to himself and others, and he's a sponge for healing potions since he can and does take enormous damage winning fights. And in one session, he was the reason they won what should have been too-tough of a fight for the PCs that made it to the session. At that point, it went from "let Raggi get killed" to "protect Raggi, he's our fight-winner."
Since then he's gotten better - better armor, a little more skills, a powerful magic axe, and points spent on more survivability have made him a peer to the PCs. The players always bring him if he's not off on a bender somewhere (he appears on a 15 or less, 12 or less if they had a big haul and he's got more drinking money). Not only that, but they've been organizing their tactics around covering Raggi while he's berserk and limiting the number of opponents who can get to him.
At one point, the group even voted him NPC, despite the fact that I don't track XP for NPCs. I bought him something with it (Focused Fury IIRC).
But how the hell is he not dead after session after session of charging into combat without armor and without real support?
How he's built
I'll be a bit vague here because the PCs don't know Raggi's stats yet.
Raggi has HT around 12-13, HP 20 or so. His weapon skill is in the high teens - enough to hit, good enough to Parry if he cares to.
Importantly, he has Berserk, but he also has Hard to Kill 2, Barbarian DR 2, Barbarian Crushing DR 2, and Recovery.
This plus a mail shirt makes it hard to deal a lethal blow to him. A mail shirt over his two kinds of DR is DR 6 - not bad, and it means he can shrug off a lot of minor attacks.
As originally designed, Raggi had SM+1 and 18 HP. Bad idea. SM+1 means healing spells cost double (cost for Regular spells is multiplied by 1+SM). But 20 HP would have given him double HP back from healing spells, making it effectively a wash. He didn't have it, so he was hard to heal. I fixed that quickly when he started to succeed enough to justify him having some points to spend.
Recovery helps, too, because he wakes up from injury very quickly - so he can get hammered down, healed, and then stand right up and be ready to go. He's not a burden to his fellows.
How he's not dead
Some of it is purely good die rolling. Raggi has had to make a lot of death checks, and passed them all mostly through luck. He's made self-control rolls for Berserk more often than not. He's got a 14 or 15 for his death check, and at least a 16 to stay conscious. This is a good because he's going to stay standing and chopping and slaying (thanks to Berserk) but won't up and die suddenly when he calms down. His main problem is that he is up and slaying until he's nearly dead. The real risk for a berserker is that you're a threat and need to be chopped into burger - so you don't take a serious wound and then drop, ignored, but stay up and get hacked to death. Even when it's obvious that just staying down is the good move, you can't.
More recently, teamwork has helped. Raggi gets lots of support from his fellows. They'll keep him back from the front and throw him into the fray once he's got clear targets. They've been putting Missile Shield and/or Great Haste on him. They've been trying to clear the area around him, and keep Vryce near him to parry for him. They're clever enough to try to keep one clear target within axe-reach of Raggi on his turn but keep him from getting swamped between turns.
Finally, Raggi isn't stupid. Berserk, yes, but not stupid. He won't put himself into a suicidal situation before Berserk happens. He'll parry, he'll dodge, he'll keep foes at reach. He (usually) won't All-Out Attack when not berserk. Although he's been getting overly confident in Vryce's ability to save him.
But for a one-dimensional berserker with an axe, Raggi's had a charmed life.