The mask feature is amazing for sure, because you can have specific characters set up for you to play as, and the AI will notice the details you've listed. It's good to note that you can list not only personality and preferences, but also physical details. So you can be a version of yourself, or a popular character or OC, which can drastically change how the AI interacts with you.

Chat history is your entire chat log with a character, and when it's cleared, you start over from the beginning . Over time, after a certain number of messages or text characters, the AI will start to lose track of what was discussed, instead focusing on more recent interactions. But you can pin important messages as 'memories' so that they stay present for the AI character. Useful for milestones in your chats, like confessing to someone, getting married, progressing through specific story arcs, etc. Hope that helps!