Bleh, got busy at work but I'll do a short breakdown.
There are two major ways to approach working as team in the game. One is to set roles (i.e. front/back) and the other is to not assign roles which is often what happens in shuffle games or free for alls. Both have their positives and negatives but generally, assigning roles is far more effective as there's a clear understanding of what needs to be done and who is managing what on the battlefield. It provides a general guideline for dealing with situations in the game as you face them whereas by not assigning a role, it can be quite a scramble. To split it a bit more in-depth
Assigned Roles -
Front: The player who takes on this position is the "forerunner" of the pair. They're the one usually being the aggressor for the team, taking damage, and helping to create/force damage situations though this will vary based on team comp and player style. They're the ones initiating contact with an opponent and are usually leading the pair during a fight. In general, they're the ones that want to be killed first to prevent a bad overcost situation as they're in the position to take more damage and to keep the attention of your opponents.
Back: The player with this role is the support of the group and follows their partner up in battle by helping them out during a bad situation (like getting melee'd) and to grab damage when presented with the chance They want to be taking as little damage as possible while doing this to keep the team stable and to prevent an over-cost situation.
No Assigned Roles -
In this case, the players will usually act as independent entities and change their gameplan on the fly to adapt to the situation. There isn't a front or back player explicitly and their movements, from a team perspective, will be all over the place. For these reasons, it's generally a very volatile play-style. With no clear person focusing on not taking damage, they can easily be overcosted. They can also be easily preyed on without a lot of discipline from the individual player because there's no guarantee that the partner is looking to cut or protect their partner, etc. This is not to say that you can't play without assigning roles but the amount of synergy between players and the skill required makes it a tough call for most teams. You'd basically have to both outplay the enemy team overall. It's certainly possible but not a good style of play for most people.
There are no set character arche-types that require you to play in either style, FYI. Even a God Gundam can be a back but it would require them to focus on dodging and looking for opportunities to get someone in the back. Like--wise, a team composed of two shooting units will have a "front" unit still that despite focusing on running away, can take a few more risks to get the damage they need.
That's kind of a simple explanation, so if anyone has any questions, please ask. It's a little easier to answer a specific question than to ramble on, heh.