Since I began reading about Danmachi world i've been asking the same question: How did Ais was born?

Information Those of a Demi Human race must mate with others of the same race to give birth to children. The only exceptions to this are Humans and Amazonesses who can bear children with any race, and half races, which can also bear children with Humans.

Human
Humans are generally said to be the weakest race.

Spirit
Spirits are said to be the children the Gods loved the most. They are strong magic users and can even use magic better than Elves. Like Gods, Spirits can't have children. There are different types of Spirits such as Salamander, Gnome, Tonitrus, Lux, and Undine.

Xenos
The Xenos are monsters that have intelligence. They are much more intelligent than other monsters and some can speak the Human language. They search for their comrades in the unexplored portions of the dungeon. All Xenos have a strong longing for the surface and someday desire to reach the surface.

So, here are my Thougths:

As Engineer i was taught to think and solve problems starting for the easier way available, so with that in mind, i'll try to figure out how Ais was Born?

1._ Homunculus... Rejected!

2._ Ais's parents are Albert Waldstein + one Human Female from his Harem. Ais was badly Sick and Aria give her Blood to Save Her! If we check both Aria and Ais are extremly Similar Phisically speaking, so i think this can't be how Ais' origins starts! Rejected!

If truly Aria is Ais' Mother - a Spirit that can't have children just like Gods and Goddesses (as we know Spirits are a copy of Gods and Goddesses), then let's get Wild with the Theories:

3._ Fetus Orb: Apparently the Corrupted Spirit (probably Aria) is using some kind of plant's DNA to get these plant-like-monsters (Violas, Viskums) + Fetus corrupting other monsters to originate a Demi Spirit. We have these resources plus The Biggest Mother of all: The Dungeon... we have Xenos Equation too!

If we take Angelo_Gabrini Theory: "Ais might be a Demi Spirit like the ones the Loki Familia's been fighting, except that instead of a monster, it's attached itself onto a Human" / I like this Theory except for the part ...Attached itsel into a Human... I think Probably Aria a Spirit Capable of make Miracles (Great Spirit) (If Fels could revive Wiene after she was only ashes on the floor, a level 4 mage with Incredible Magic can do this... so how about a Spirit who's a copy of Gods and Goddesses) could just take some DNA from Albert and some DNA from herself make a Fetus out of it and the Dungeon (a Wall/Pantry)  acted as a Incubator...? i think it could work this way! or... (This Theory is the one i like the most)

Aria took the Fetus Orb and Attached it to a Human-Female like Angelo wrote or even attached it to a Xenos-like-Human-Female like Wiene. Rejected! Ais is growing like a normal Human Being.

Other Possibility with the Fetus Orb (Fetus originated from Albert's DNA + Aria's DNA) is:  Albert and Aria put the Fetus Orb into a Pantry (just like Revis does with the Fetus that originate Demi Spirits) + Spirit Magic (a Soul fragment from Aria is put into the Fetus, we know that the souls from living things can be fragmented), and This Way Ais was born. Sound Interesting!

4._ Perhaps Aria assimilated a Human-Female/Xenos-Female and give her the Ability to get Pregnant! o.O Rejected!

5._ Conclusion: I Personally think that Ais is not 100% Human.

._If Fetus Orb is involve + Dungeon: 50% Human + 30% Demi Spirit + 20% Monster

._If Fetus Orb is involve + Human-Female/Xenos-Female: 60% Human + 20% Demi Spirit + 20% Monser. Rejected! Ais is growing like a normal Human Being.

._If Albert's DNA + Aria's DNA are involve + some kind of Miracle from Aria's Power: 60% Human + 40% Demi Spirit.

._If Ais was conceived the "Natural" way using an Artifact Power made by Gods allowing Aria to get pregnant: 50% Human + 50% Demi Spirit.

Only Time will give us the right answer...