I suggest the following:
Players can designate themselves to be the bodyguard of one other player at the same time. That player to protect may not yet have another bodyguard. Once they sign up to be the bodyguard of player they are no longer bodyguard of any player they were bodyguard for before. Players can fire their bodyguard (which means that player is no longer their bodyguard (so someone else can be), no other effect. A player can also stop to be bodyguard any time by choice.
On the profile page is shown if there is a bodyguard protecting this player and who it is.
If you attack a player that has a bodyguard and that bodyguard is in the same city as the guarded player and is not in hospital or jail, you fight against the bodyguard instead.
If you do not win that counts as successful defend, run-away or stale-mate of that bodyguard and the fight ends (just as if you had attacked the bodyguard).
If you win you get an successful attack logged, the bodyguard gets an unsuccessful defend logged and you get to chose either one of the normal actions (leave, mug, hospitalize) against the bodyguard or one new option "continue".
If you have won against the bodyguard and chose the "continue" option, the bodyguard gets into hospital for the same time a "left on the street" would cause but no other effect happens for the bodyguard, and you are into a fight with the original target (without needing additional energy). The result of the fight against the original target is handled just as if you attacked someone without a bodyguard or a bodyguard not ready.
Hoped for outcome:
I guess people would usually expect payment to become bodyguards. As there is no way to enforce this payment, bodyguards might be bribed to give way (or just start being bodyguards, accept the payment and then stop being bodyguard). Also bodyguard will likely require payment up front to not be scammed, either. So this would require trust just the same as many other things currently do. Some people might start to bounty body guards to be able to attack the one guarded by them. Healers might want to look out for their bodyguards and heal them when they get into hospital while offline so the healer stays protected. Some people might explicitely target guarded players as they get twice that successful attacks for the same energy (but might be overconfident, entering the second fight already wounded and thus give the guarded player some free defends). It might have all forms of interesting effects on faction wars (might allow factions to accept some lower level people and look out of them, switching guarding them between each other). In the end I hope that also allows some ways of playing that do not have to focus on fighting stats so much (but instead then have to focus on managing having a bodyguard ready all the time).