Here are some house rules I've used to great effect – based on the writings of many bloggers and forum posters.
Simplify casting times to Short rest and Extended Rest
If casting time is less than 1 hour, make it a short rest instead. If it's longer, make it a extended rest.
Severely cut the casting cost
Cut casting cost by at least 50%. Consider cutting it by 90%, and eliminating the cost completely when less than a threshold, say 10gp.
Make disenchant free and allow residuum to replace material components
By making disenchant free, the residuum component cost can easily be satisfied by those old junk magic times cluttering up their character sheets.
Alternatively: Replace all material component costs with a per-day limit
Instead of having any residuum and material component costs, some GMs are implementing a per-day ritual casting limit: say 3/4/5 for Heroic/Paragon/Epic. Any rituals that are too powerful this way are simply not available to player characters.
Use a skill check to allow casting a ritual above level
I love low-level characters being able to cast rituals/create potions/etc. above their level. Invest the time and the component cost, if any, and roll the appropriate skill check. Difficulty starts at 10 for a ritual/potion one level above you, and goes up by 5 per level higher.
For the inspirations for these house rules, see:
Focus on one pair of characters
A lot of text-based roleplay with romantic or erotic content focuses on each player running one character at a time, even when each player has a whole stable of characters they play intermittently. Likewise, such roleplay often focuses on one-on-one scenes, even in a game with numerous players. It sounds like the two of you wouldn't be interested a story like "our hero sleeps with everyone they meet between adventures", anyway. So I suggest that you have each player character romancing one NPC.
When you focus on developing these two characters, it should be amusing to see how they relate to your own personalities: Without the characters merely being stand-ins for yourselves, you can still emphasize traits you find endearing about each other... or you could "play against type", since a lot of charming heroes in fantastical fiction have traits that would be truly obnoxious in a real-life partner. The focus on two characters also brings us to my second point...
Build up the tension until it's unbearable
This is a factor that people have been considering in text-based roleplay with online acquaintances for decades. Lots of people love a sexy subplot; lots of people also don't want to seem like they're just in it for the adult content when everyone else is really into the fictional world and characters. So it seems "impolite" to push things in that direction too often or too firmly, and the result is a lot like a "will they or won't they" plot in a TV show.
This can work by adapting formulaic stories that are obnoxious in other types of fiction. In TV, film, etc, a character constantly trying to win another's affection often makes that character seem like a jerk. But when roleplaying with an in-character relationship in mind, you both know that the object of their affection secretly reciprocates those feelings. Likewise, this can work with two characters who secretly like each other but are afraid to admit it - as an audience member, you may be yelling "just kiss already!", but while roleplaying, you're building up how that happens in a way that will be satisfying for you both. This kind of thing is eminently compatible with a serious plot - the back and forth between them builds up over the course of the story, until they finally admit their feelings and fall into each other's arms.
This buildup of tension can easily be driven by circumstances rather than characters' personalities. For example, let's say your character and the NPC they have a crush on are planning a heist together. No matter how much they flirt, they're probably not going to act on it for a while. It may be inconvenient to make a move while they're reviewing maps and buying smoke bombs. It could cause a lot of trouble to steal a kiss while they're picking locks or sneaking around silently. And it certainly ruins the mood when they have to escape through the sewers. But they can be flirting the whole time... and the love scene at the end of the adventure should have that much more impact, after nearly getting distracted from the mission because they were both thinking about how cute the other one was.
But there's certainly also a lot of more sophisticated ways to...
Give them a reason that the relationship is inconvenient
Establish a concrete reason that they can't just fool around all the time, one which fits the tone and setting of the RP. The heist plan in my example above is one that works to defer that side of the plot until the end of an adventure, but you can come up with a good reason that love scenes can only be intermittent throughout a longer storyline.
Maybe they're on opposite sides of a conflict, working together to face a greater threat... and their allies accept that, but publicly revealing their love for each other would be scandalous. Maybe a casual relationship, without a proper courtship, is just seen as improper in their society. Maybe it's a comedic matter, where something explodes or a crazed barbarian bursts through the door as soon as they're getting their clothes off.
The pacing of this may be too obvious or monotonous for you, or it may be a bit of a letdown once the characters get intimate and the built-up tension is relieved. In that case, you might want to insert some more pointed conflict into the romance, and...
Introduce a love triangle
Sure, I still think the focus on a single couple is the best way to proceed, but you're not writing an epic of star-crossed lovers. At some point, there's a reason to break things off - it could be a conflict like the ones described above coming to a head, or a the player character might make a bad decision or a bad dice roll that offends or disappoints their NPC love... or they might just be heading out on separate adventures. That's the opportunity to introduce a new NPC with hearts in their eyes, or an existing NPC with newly revealed affection for that PC.
This is a way to "reboot" the flirtation and the buildup of tension, and introduce a new obstacle. The PC may only have eyes for their prior love, while the new NPC does everything they can to win them over. Or plot-based conflicts between the three of them may make the whole affair a risky flirtation. Or they could party up together, while the PC's split affections cause turbulence in their adventuring. If you're not planning on true love winning out every time, it should be a lot more fun to periodically complicate matters this way.
A lot of this advice could be simplified to "do what works in fiction", but it's important to consider how fundamentally different these plot devices are in a collaborative story. The key thing here is, since you two trust each other and are on board for this kind of roleplay, you can look ahead at the results you're aiming for. Ask how you can set up the type of relationship that you find engaging in fiction - and how you can avoid the cliches that annoy you. When you know what that goal is, you can use those plot devices, alongside the narrative and mechanical progression of the game, to find out how you get there.
Best Answer
I find that answering the question "What would $PCName do in $situation?" work well. Come up with many different situations and answer them as your character would. If that's too complex, make sure you pick two or three characters in fiction you like. Those are now your character. Now, you should be able to answer the above easily enough. That should give you a base line that you can use to role play your character.
Look at how authors deal with their character. This is the best way to learn how it's done. Pick good authors (or/and authors you like) and look at how they build their characters.
Finally, the more flaws they have, the more interesting their story is. Gandalf was scared of Sauron because he could see himself falling to the same patterns. Dr Bennan (Bones) deals with cold science because of her abandonment issues. Spike (Cowboy Bebop) refuses to admit he has friends because he hates himself. You can spot the pattern by now. On the other hand, Superman (for me) is utterly boring: invisible, immortal, and can solve anything by punching it really hard... Yawn ;>
Overall, think: What would my character do to make a better story?