Explorer/Cygnus is one or the other. Not both. You gain one attack/ magic attack for every other character on the server for every five levels your Cygnus gains, and every 10 levels your adventurer gains.
Cannon Shooters give +15 all, and +5% HP/Mp
Mercedes give 10% Bonus Exp per kill. That is not factored into Party Bonuses, but takes into account Holy Symbol.
Demon Slayers give +10% Boss damage. This stacks with all other potential.
They only work for on whatever the server the character is on. So a Mercedes on Windia can't link to a character on Scania, for example. Your Cygnus or highest level Explorer gives an attack boost to every other character on that server. Cannoneer, Mercedes, and Demon Slayer can only give them to one character (not including themselves) at a time, with being able to change the character once a day.
Cannoneer -> +15 all & +5% HP/MP Mercedes -> +10% EXP Demon Slayer -> +10% Boss Damage
Those are the current ones. When Phantom comes out, he'll have one that gives extra Critical percentage. One character can have every skill linked to it at once, but not a double of the same skill, so no +30 to every stat or +20% EXP
PSN: gamesage53 Tell me you're from here and the board you were on so I know to add you
for every 10 levels, your highest leveled character gives all other characters +1 ATT (and those other bonuses no one cares about) for a max of 20, but a Cygnus Knight gives all other characters +1 ATT every 5 levels for a max of 24, the higher bonus between the two is what gets applied (characters do not get their own blessing though)
Soul Links however, can only be applied to one character at a time, and changed once a day, although the characters that give them will always have them as a passive
in addition to what gamesage53 said, the upcoming Legend Phantom has a Soul Link that gives +10% crit chance
and of course, there's the newer Character Card system, which gives you even more bonuses for making multiple characters: