Character Relationships: Spatter Painting with Purpose

   This month's prompt:
      Character relationships.
         Oh joy- this is going to about kill me.
     Bonus Points: Give us a picture of your planning process when it comes to characters!
         I'm going to do the easy part first:

Spatter painting with Purpose!

       How hard is it for you to create character relationships? 
         This depends entirely on the character. I mean, All characters (bar hermits, psychopaths and sociopaths) have relationships of some sort. The majority of my characters begin with facsimile relationships because they themselves are still "flat" characters when I begin writing- as they grow into themselves, their relationships become more dynamic and believable.
         Interpretation- insultingly easy to absolutely impossible.

       Do you pre-plan them or do you end up letting them develop as the novel goes on? 
         Some relationships I build into the framework of the story- best friends, Main Character love interests, mentors,  how they relate to their families... The rest just sort of spring up along the way.

       Have your characters betrayed you and ended up with someone you didn't expect?
         You had to get me started. I have this one character duo who have taken this to the level of headache inducing. Saeki and Tsora. They're elemental mages from rival clans who are roommates at boarding school. Neither of them has a good home life, and they have a really odd exclusive relationship. The best way I can describe it is that they're in an asexual relationship with one another.

      Give us the story of how your favorite written characters got together and what makes their relationship the strongest.
      Confession time! My favorite written characters are not both mine. They inhabit a story I've been working on with my writing buddy Kinkatia for over a year. Dan Mandel (originally Dan Mandarion from a Star Wars fan fiction I did with another writing buddy, Teva, when we were thirteen; which we then proceeded to rework at least four or five times over the next eight years) was lent out as a mentor to her character, Sebastian Moran, in the art of being a freelance assassin/bodyguard. We found their interaction so interesting that we traveled back in the timeline to write the back story for their relationship, and then an alternate ending to Kinkatia's original story.
      In the beginning, Dan picks Seb up after he was wounded in a knife fight and objects to the man dying in his kitchen; Seb survives and becomes Dan's apprentice, then partner (professionally). They separate and work individually for some time, during which Seb ends up in a relationship which ends tragically when his partner suicides. Dan picks up the pieces and they go back to being professional partners for another several years before Seb's little sister forces the issue of their 'feelings.'
      *phew* And that was the short version. The story itself is well over 150k, and still in progress, as well as having spawned its own AU in which the evil partner (Dan's opinion) never existed.
      This was originally a BBC Sherlock Fan Fiction, but we may, with a little tweaking, be able to have Dan and Seb stand on their own as an original work.