Howdy-howdy! My starter from years and years ago is a sweet gal, I attempted to remake her when I was given the regen prompt to spice her up, but.. I'm fairly unhappy and disappointed with myself at the result. I really should've taken more time and got more experimental with her colors. She'd be so much better with more additions, you know? But I was limited to only so few.
Is there any way to give her a more detailed makeover, and maybe even change the species? Since she's unable to sold or traded as I've been informed, I would release her, but it appears that she's crucial to my account as as her level increases, the level mentioned beneath my username also increases in conjunction. We're currently at level 45, but I have purchased two very lovely level 60 pets, only to discover I cannot use them in an active party as my level was insufficient. So it definitely seems like she's crucial to my account's existence and development.
Could I purchase a Mythical Essence and use it on her or generate a new pet with it and put it on her ID to serve as my 'new starter'? Orr perhaps a different solution?

Well...you can use pet dyes (change colour of pet). But you can't use the genetic mutators (give genes and mutations) can only be used on captured pets. Other than that, you can't really do anything...

You aren't alone, btw, you should see my starter, it's completely white cuz i got confused when i started

Howdy-howdy! My starter from years and years ago is a sweet gal, I attempted to remake her when I was given the regen prompt to spice her up, but.. I'm fairly unhappy and disappointed with myself at the result. I really should've taken more time and got more experimental with her colors. She'd be so much better with more additions, you know? But I was limited to only so few-

Traits available to regen pets are restricted to those that were available pre-revamp

Is there any way to give her a more detailed makeover, and maybe even change the species?

Aside from pet dyes, not really. It is a possibility that when the site is eventually relaunched we'll be given a chance to alter starter pets, but that is only a possibility, a bit 'maybe'.

Since she's unable to sold or traded as I've been informed, I would release her, but it appears that she's crucial to my account as as her level increases, the level mentioned beneath my username also increases in conjunction. We're currently at level 45, but I have purchased two very lovely level 60 pets, only to discover I cannot use them in an active party as my level was insufficient. So it definitely seems like she's crucial to my account's existence and development.

Your account level is not tied to your starter, but rather is dependent on the highest level you've raised a pet to. If you had another level 45 and raised them to level 46, your user level would rise.
It merely seems this way to a lot of newer/less active players because starter pets tend to be the pet people use in all their early battles, putting it ahead of wild pets in the areas you fight in (unless you're in a fest zone) so any pets you catch will start at a lower level than it.

Could I purchase a Mythical Essence and use it on her or generate a new pet with it and put it on her ID to serve as my 'new starter'? Orr perhaps a different solution?

This is not how Essences work- they cannot be used 'on' pets, and are solely used to create new pets in the generator, and no pet can 'overwrite' an older pet's ID.

most of the question were answer but for the mutator, while it's thrue that you can't add any gene on your starter nothing prevent you from using a restricted mutator to change at rendom one of the trait and hope for the best.

Ohh, I see! So the reason why I can't use my level 60 pets that I purchased at 60 is because the most recent level I've risen is to 45. If it had been any other pet that I owned with a level below my personal cap is leveled up above 45 in my own hands, it will raise the cap by itself. Not necessarily the starter, but the one I've personally trained. Gotcha!
That clears a bunch of stuff up, thanks everyone! (And as for the Essences bit, I'm aware of the Generator but wasn't sure if it has a secondary ability of being able to overwrite an existing pet's appearance, like the customization and heir features on Lioden haha)