Explanation: The Praetorian Guard of the Roman Empire were the main bodyguard unit for the Emperor. They were recruited mostly from Italian natives, and largely used as a secret police. For this reason, they developed a... nasty tendency to extort and even overthrow insufficiently 'generous' Emperors. After all, when you spend all your time seeking out the kind of people who are likely to overthrow the Emperor, and how they whisper, in what euphemisms they use... you also know all the players who could be involved in overthrowing the Emperor... if you, theoretically, developed a taste for it yourself...
The later successor state of the Byzantine Empire, centered in Greece, also had several bodyguard units, but the most famous of them was the Varangian Guard. The Varangian Guard was recruited from Germanic peoples - initially Norse, but later including large numbers of Anglo-Saxons and Germans. Despite being recruited from foreigners, they were famously loyal. This had multiple causes.
As outsiders, the Varangians had no loyalties nor really any safety other than that which was granted by the Byzantine Emperor. No family, no old connections, no longstanding economic interests - all that's back in ~~Germania~~ Northern Europe, far away from the Byzantine Empire and its power players. They're a bunch of fellows who can barely speak Greek, much less subtlety and implication in courtly language! No one knew them, and they knew no one, effectively - meaning throwing their lot in with a coup would always be a very dangerous prospect. No one is easier to get rid of than the foreigners who no one will mourn the departure of. Traitors are always easily disposed of, because no one knows what a traitor will do next - they're always a threat.
Germanic cultures of the period placed a very high value on personal and contractual loyalty. Christian or Pagan, the oath was always binding! Gift giving and other such personal connections were also more prominent in Germanic cultures than strict exchange. "My chief is good for it", sort of energy - the fulfillment of the agreement is not so important as the expectation and effort to fulfill it. And after all, when you're short on one gift, aren't you twice as generous with the next to mend any potential doubts about your trustworthiness? What's waiting another few months when you get your rewards with interest?
There was also a strange tradition of allowing the Varangians to loot the treasury and take all they could carry whenever the Emperor died. This might seem counterintuitive, but it was actually extremely effective. What's a missed paycheck or two compared to THAT long-term benefit? Just a few more years, Bjorn... the old Emperor's heart will give out soon!
Not only that, but it also avoided prolonged Varangian involvement in Byzantine civil wars. By letting their oaths expire upon the Emperor's death and paying them off, the Varangians have no incentive to fight to the death out of desperation, 'avenge' their employer, or take up personal feuds against the new rulers. Both sides shake hands, the treasury is emptied a little, and the coup gets to be successful - just as soon as the old Emperor is killed, and not a moment before!
That also made Varangians highly trusted, because while they were clearly very mercenary, they were predictably so, and that predictability was highly valued in the cutthroat politics of the Byzantine court. Sure, they'll be your enemy, up to the death of the Emperor, and not a moment later. Any of them who re-enlist in your service can be trusted completely, or close to it. They fulfilled their oath to the old Emperor, and they'll fulfill it to you with the same single-mindedness, so long as the same terms are offered to them! And what do you care if the treasury is looted after you die? A small price to pay for a loyal bodyguard!
So the Varangians got to be immensely trusted, simply by doing their best to the absolute letter of the agreement - they serve the Emperor with their lives, for as long as the Emperor lives! Job security, end-of-term bonus, and no hard feelings from any putschists who do manage to win - not a bad deal! And after their oath expired, they'd take their perfectly-legitimately-obtained wealth, and be free to roll the dice again in Byzantine service, or hop on a ship and go back home to drink mead and share tales of far-off Mediterranean shores with their hometowns, wealthy men all!
♪ The axe-bearing foreigners they have aptly named us; O! We come from afar... ♪