However, it was later found out that Manas can be born when any form of inanimate and intangible existence develops a will and ego, and are able to maintain their own existence in the form of a 'core'. As such, they are more accurately a kind of 'will harborer' or 'will carrier'.
Thus, it is natural for them to adopt the will of their original host; a kind of 'inferior psychological clone, that have high resemblance to their original host, but are still very different and cannot be considered that same person.
Manas is a very unique existence as it can be considered a combination of a spirit, a pseudo soul, an ultimate skill, an individual character and an ego; or non at all.
Process of Birth
There isn't any concrete and definite 'process' for the birth of a Manas to take place, but simply a collection of conditions that have to be met. It may be conjectured that the three main things needed for their birth are, 'ego/personality', 'will(s)' and 'self sustenance'.
There isn't any particular order in which the first two conditions need to occur, but usually, it is more common for the ego to sprout before the will.
It is unknown if Raphael and Michael are special Ultimate Skills who can act as such and talk independently like some kind of A.I., or it is possible for just about any Ultimate Skill to have their owner's personality 'rub off' on them with extended periods of time together, thus turning into this kind of talking, self-acting A.I.
If the former is true, then Michael and Raphael were made special and it was simply the personalities of their original masters that rubbed off on them. If the latter is true, then the two of them developed into such special kinds of Ultimate Skills because of the time spent with their original masters.
Regardless, in due time and circumstances, this developed ego can play a crucial part in driving their thought processes. In this stage, they are still purely logic driven Ultimate Skills that only exist to serve their masters, but certain behavior similar to human emotions may surface from time to time.
The other type of development known is when an individual bottles up enormous amounts of their emotions or has a surge or outburst of it right when they die. It is also likely that the particular individual needs to be a powerful existence as a prerequisite.
In such a case the emotions stored becomes its own version of the original individual's ego and also plays a huge part in deciding which will of the original individual the Manas will carry.
So far, the only known method for evolution after this stage, is naming. Apparently, granting a name upon an Ultimate Skill solidifies their individuality and ripens their ego. A separate physical body isn't necessary. The newly formed Manas can live within their user as a spirit. Kind of like having two souls in one body.
Since it is the core of the Ultimate Skill itself that has been nurtured and evolved, it automatically has self sustenance. Afterwards, it stops behaving like an Ultimate Skill according to the laws of the World and is treated like a Soul. Meaning, that once a Manas (or its host) 'dies', they are gone for good (although it is unknown if they also follow the rules for reincarnation), unlike Ultimate Skills that are simply 'returned' to the World until a new individual obtains it.
Moreover, once the Manas is formed, they separate from their Ultimate Skill shell and become a separate existence on their own. Unlike conventional skill evolution, where the integrated/fused skills are erased, here, the Ultimate Skill isn't discarded. Both the Manas and the Ultimate Skills retain the same skill effects, which is basically like having two of the same Ultimate Skills, except that one can think and feel like an actual human.
This is proved when Manas: Ciel discards her former self (or 'body') Wisdom King Raphael, as it was integrated with Gluttonous King Beelzebub (along with Storm King Veldora and Scorch King Velgrynd) to create Void God Azathoth. Raphael was erased, but Ciel remained, along with the skill effects.
Even if they're kinda the same, the Manas is obviously higher spec than its respective Ultimate Skill. One important factor is that they no longer think through pure logic; their thought processes become more flexible.
Things are more complicated for these types, especially for Manas: Chronoa since nobody technically 'bestowed' a Name unto them. While it is somewhat understandable for Manas: Velda to be called 'Velda' since his original source is Veldanava who had renamed himself to Velda Nava, before he died, it is completely mysterious how Manas: Chronoa obtained the name 'Chronoa'.
The only possible reasons would be an untold scene of Chloe naming her, the World itself attempting a correction mechanism and naming her in relation with her nature as a Spirit of Time (Chrono-), or she named herself.
The will carrier of the late Lucia. She bears a mischievous yet kind personality and holds deep loyalty and gratitude to her second and current master Rimuru Tempest. In the Epilogue, she even considered herself Rimuru's default official wife while Milim, Shion, Shuna, and Chloe were bickering amongst each other for the position.
The will carrier of the late Veldanava. Even if she carries his will, she still considers herself and is aware of the fact that she is different and that is his servant rather than the being himself. She holds enormous grudge towards the World that killed its own Creator and wishes to destroy it as sacrifice to recreate and reunite with her beloved master Veldanava and his wife Lucia.
Also the will carrier of the late Veldanava. He holds all the negative emotions and grudges that Veldanava had for the World for rejecting its own Creator. Unlike Manas: Lucia, Velda considers himself to be the core of Veldanava himself as he has the same memories and also emits the same aura as him. This was why Dagruel followed through with his orders even if he was aware that Velda was not the same as his master Veldanava.
The will carrier of Chloe O’Bell. She also holds all the negative emotions of her host, similar to Velda. Every time Chloe failed to save her loved ones in each time leap, her frustration grew and personality changed. However, after obtaining the Ultimate Skill Hope King Sariel, she forcibly sealed all those emotions and continued to leap back repeatedly. In due time, it grew to such an extent that confined emotions was born as an evil Manas that is Manas: Chronoa.