Yfa's Mama, as shown on the Cast page:
Of course, her real name isn't Mama, but her actual name is not revealed here.
Mao thought all the Gods died long ago, which is presumably common belief (Fahrin also details how the Darkness God died later, so many of the Gods probably are dead for reals):
But as Kalliv mentions, the Tree of Life hasn't withered yet, therefore Methuselah must still be alive:
Trèfle later also mentions that Methuselah lives at the Tree of Life:
So people outside of the Forest Clan believe she is dead, despite people in the Forest Clan believing she is alive. Of course, it would be evident to everyone that she is alive if people still encountered her, so presumably she is either rarely/never seen, or rarely/never recognized. If she is living in a different form to her usual, then it would of course be much harder to recognize her.
I posit that Methuselah is living in the form of a beast, as Mama.
Everyone refers to her as Mama, rather than her actual name (Methuselah), as she doesn't use that name in her current form:
Like how the Tree of Life would wither in Methuselah's absence, Methuselah (Mama) would wither in the Tree of Life's absence. And so, she returns to the Tree of Life to live there:
Bae understates how strange her family is, with the children being the offspring of a Spirit and a "Fruit Fox":
The strangest part of course is that Spirits are Not Born That Way. Spirits are born of emotions (links here later) or from a Spirit dividing itself into separate Spirits, rather than by any normal reproduction methods. Fruit foxes presumably do reproduce similarly to irl foxes, which is vastly different to Spirit reproduction. They should be thoroughly incompatible, right? So, something unusual has to happen to have a half-Spirit half-Beast child.
Chris explains that "a ritual to create life was needed for the pregnancy" and "There isn’t a lot of half spirits because it’s a lot of work". So some sort of magic was involved in order for them to have children, that is only ever rarely used.
The Spirit aspect of reproduction - "Papa has to literally give pieces of himself away" - still applied here from Papa's side, as well as a pregnancy (which I've been reliably informed is hard work irl) from Mama's side, so clearly things would have been a lot of work even before compounding that with a magic ritual on top of that to make it even harder.
But then consider that "There’s 5 of them in this family alone because they really have a ton of love and really wanted to make it work" like wow, first of all they must really love their kids to go through all that for them, but also, something so hard to do normally once would probably be much easier to do five times if one of the two involved was a God!
Yfa also gets a strong feeling that he has to visit the Tree of Life upon seeing it - a strong enough feeling that he is fine to put off searching for Yokoka (who he'd been looking for for weeks/months already after disappearing after her fight with Hurricane):
I think Yfa was feeling that his Mama was missing him (he'd been lost in Betel's Forest for two years); it would make sense for him to be as overcome with a feeling of wanting to return to her from such a distance if she had the power of a God!
Yfa also very quickly regained his vibrant palette within a few days of returning to the Tree of Life and eating his favourite blue heart-shaped apples there. And also... gained the ability to use plant magic without anyone actively teaching it to him:
We've seen other people learning (and failing to learn too) magic before, but this is different in that he pulls it off with no advance warning or practice, no incantation, and it's an entirely different type of magic than he's used before. While maybe this could be a more innate/natural type of magic (like shapeshifting for instance) than other forms spellcasting, it's also worth mentioning:
Kalliv specifically mentions that Methuselah "lends us her powers", and this new plant magic being power lent by Mama would explain how Yfa mastered it immediately. It also matches the element of the Forest God, and she would've been in the perfect position to grant that power to someone living in the same home as her for a while, and of course would want to do what she could to help protect her son leaving on an adventure.
It's also worth mentioning that there is precedent to lending powers to someone else, Copy lets Fahrin borrow one of his eyes, though as Mao notes "people don't work like that", and while he calls it a spirit skill it is probably also within the power of a God.
Relatively minor note compared to the rest, but still worth mentioning: Kalliv refers to Methuselah with she/her pronouns for the entirety of his explanation about her. The biblical Methuselah was a man botanical Methuselah is a tree, so the use of she/her here would be an intentional choice (Kalliv knows those are Methuselah's pronouns), as opposed to just defaulting to he/him it/its based on the name. [Edited to account for Methuselah being a
not a 
]
And off course, this matches up with Mama's pronouns:
