How should Mystia's nickname be romanized?

A fan nickname of Mystia Lorelei is みすちー or みすち. It's interesting because it sounds just like an English hypocorism, even though it was probably came into mostly because ち is easier to type than ティ. It shows up in commentaries and sometimes in comics, so for the purposes of translation, how should we romanize it?

In the tag "Okamisty", it's spelled "Misty". This is bad because it's inconsistent with the character's real name, of which this nickname is only a minor alteration. It could also possibly cause confusion with the other Misty, even though that one is called something different in Japan.

In our translations, the most common spelling seems to be "Mysty". There's no reason for this to be incorrect, but to my eyes it just looks wrong. "Mystie" feels much better. It's how the name would probably be spelled if English speakers had come up with the nickname.

A fourth possibility is to romajify it directly as "Misuchii", but that seems a little silly.

Thoughts?

The Touhou wiki romanized it as "Misty".

Even though the "ti" part of "Mystia" is supposed to be 「ティ」, many fans have found it more convenient to represent it by chi (ち) instead. Thus, Mystia's name is commonly abbreviated to "Misty" (みすちー or みすち misuchi) instead, which has become a nickname of sorts.

I'm not too familiar with the translations of fanworks, but i believe this is the most used translation of the nickname.
I'd suggest against making a term that only Danbooru will use.

Oryuu said:

In the tag "Okamisty", it's spelled "Misty". This is bad because it's inconsistent with the character's real name, of which this nickname is only a minor alteration.

"Mystie" feels much better. It's how the name would probably be spelled if English speakers had come up with the nickname.

I'm pretty sure that English speakers came up with the nickname "Misty" for Stoneforge Mystic and it is almost uniformly spelled as such. "Mystie" looks like a misspelling and not a nickname.