Hi! Lemme know when you're here and if you want to just be in a partnership with you having one character and someone else having one character. 
I don't have to be the one coordinating the partnerships, and once y'all's characters start writing it's just completely out of my jurisdiction, though obviously let's keep the letters within Boardhost rules boundaries. So, if you want me to partner you up, let me know. Otherwise, feel free to just reach out to someone and ask them if they want to do this...thing with you. xD
Also don't feel like letters need to be particularly long or responded to quickly--I'm usually under the impression that letters take a long time to receive and send, but obviously we have the power of internet on our hands, so it's almost instantaneous whenever someone gives a response and we look for it. Nonetheless, maybe you can keep the person on the other end waiting a bit more if you have life stuff going on , it's chill. But please respond within 2 months. It's rude to make someone wait for 2 months for a letter you received within one.
Generally, we'll go by general characters that you have who have pages. If you want to make a character a possible candidate but they don't have a page, make them a page! Look at the total number of characters (who have pages) that you have and then use a random number generator. It doesn't matter which one you use, but please only click once unless you have special circumstances (like if I got Qwerty I'd have to roll again; it happens to not have the ability to handwrite letters, being a robot). It's best that you first exclude those with special circumstances first, though. Once you get the random number, count down the list to that number, and you have your participatory character. Please note it's not always the same character; for instance, if you're participating in two partnerships, you'd probably have two characters, unless you rolled the same number. Here's a random number generator: https://www.calculator.net/random-number-generator.html. 
Characters can be categorized a bit for logical purposes. For instance, it might not be optimal for evil entities to interact with good ones via letters, so if you and your partner choose to do so, you can limit your choices by that too.
This entire thing may or may not be canon. It's up to you (and your partner) to decide. On the plus side of being canonical, they could've technically met each other before meeting physically. Maybe some relationships could be borne under pen pal relationships, kinda like if we met online first and then met in real life someday. On the downside, sometimes it doesn't make a lot of sense for them to have arranged to be pen pals, but heyyyy, this is for fun xD.
Last but not least, for now, Pen Pals will be kept within this topic--Fun. Please mark your titles with something related to Pen Pals and the names of your characters who are participating in that particular pen pal exchange once you have them down. I'm not sure if users can change the titles of the posts, so if the admins and mods could help me out there, it'd be great. Normally it'd be optimal to first know who you're partnering up with (both the RPer and the other character) before you make the post, but sometimes people do things differently, so yeah.

-Galaxian-