Pre-Established relationships between characters is something coming into play with more systems. The understanding behind it is easy enough to figure out: if the PCs know each other already it is a lot easier to get the party together. As a GM I've used this and benefited. As a player I've done it. Still, it always makes me wary. Today I want to talk about why.
Making The Plan
Before the game begins, everyone makes their character. When you make the character, a lot of people - myself included - explore who the character is in different ways. We think up personality quirks, backgrounds, and how the character acts. Some friends of mine plot out a "normal day" others will answer a slate of questions. My writer friends will write little character exploration pieces. The point is, we figure out who the character is, and from that we think of how they will be and who they will get along with.
If I'm making a hacker with high intelligence and low wisdom, and you're playing the cunning charmer who knows everyone and always has a scheme, it can be easy to think of how these characters could get along and be friends. We may even plan on it. Heck, maybe we want the bond to be strong and even go for a romance angle. The characters are lovers, or married. It's great and leaves a lot of hooks to use going into the game. Except...
First Session Is First Meeting
No matter how much work I've seen put in, I've yet to meet a player who didn't learn something new about their character in the first session. Frequently I see people caught off guard by how the character plays compared to how they were envisioned. Something about being thrown into the crucible of a world not under your control, with 4-5 other people just as important as you and the needs of a game and getting things going just pushes characters in weird ways.
Recently I played the first session for a high intelligence, low wisdom hacker/mechanic. I had a strong idea of who the character is in my mind, but after session I realized that one of the character's big personality flaws is they think they're smarter than everyone else. This never even entered my imagination in planning, and it shapes a lot of how the character acts. Which in turn means that - even though the heart of the character is what I want - the character expresses themselves and acts very differently than I thought they would before. They still just go with the flow, but they're more petulant about it (though someone called it snark so I guess the Charisma is working. Why is this a problem? Emergent gameplay is good, right? Well, yes and no.
Why Are We Together?
Family is a safe bond. Why? Because you can hate family, but they're still family and people will understand. However, when it comes to Friends and Lovers you actually need to like them. People don't hang out as friends, or become lovers, without that. See the problem?
If not, let's go back to my Hacker/mechanic. We've set it up so our characters are lovers or friends. You have a Brad Pitt esque charmer from Ocean's 11, I have the "can do anything with electronics" hacker/mechanic. Only, look at what I said. In the first session I'm not coming across as withdrawn, I'm coming across as petulant and whiney. The character causes problems. They lack certain drives. They think they're smarter than everyone. You find through the role play that your character...they don't like my character. In fact, they find them to be the most annoying thing in the world. So, why are they friends? Why are they together?
It's a hard question to answer. Maybe it is a new relationship, but odds are if you planned having a pre-established relationship there is a history already there. Now you're stuck in a relationship your character doesn't like or want. So what now? Breaking it off means changing two characters. Not to mention all the other players started with those 2 being an item factored into who they are.
So stick with it right? Except, that will cause problems too. Trust me.
But I Want A Pre-Existing Relationship
That's natural. I'm not saying to not do it, I'm saying to be careful. If you do it, make sure it's a player you know and have played with before. Someone who you know the trends in their characters and are fine matching up to it. Also be sure to stay in communication with them and discuss the characters to make sure they work.
If you can't do either of those two things, then keep the meetings in the past brief, or with a gap of time in between. Dresden Files is good for this, you have past adventurers with the other characters but other stuff could have happened. The character could have changed.
Add More After Session 1
The alternative is to leave room for it. Discuss how it could be, then see how the characters are in the first session. Do they have that chemistry? Then go with it. Are they like oil and water? Don't. The important thing is to know it could work as much as possible before going into it. Otherwise you just end up with an unfulfilling RP experience.