Undertale is based on Hebrew mysticism and occultist symbology
Toriel = Turiel, a fallen Angel
Metatton = Metatron, the most powerful being in Heaven, only second to God (actually El) itself. Interestingly, Mettaton is the last boss before you fight Sans/Asgore.
Asriel = Azriel, an Angel/Demon of Death (keep in mind how Asriel is called the "God of HyperDEATH" when he assumes his final form). Also anagram for Sariel, the son of Lilith and Samael who is also related to Metatron. The full name of Asriel, which is ASRIEL DREEMURR can also partially serve as an anagram of "ISRAEL REDEEMR", but it can be one for "SERIAL MURDERER" too (which shows an evil background behind this character). In the neutral route, Omega Flowey (Asriel) highly resembles a Seraphim-like (a very powerful kind of Angel that looks like a Lovecraftian creature and can twist the reality) being.
Finally, the Asgore character holds strong resemblance to Moloch, a forbidden ancient Canaanite/Carthaginian demon god described in the Bible that demanded child sacrifice and was represented as a huge bull with big horns. The name "Moloch" or "Melekh" means "King" and what is Asgore? The KING of the Underground. Asgore uses fire to try to kill you and kids that were used as a sacrifice to appease Moloch were BURNED in FLAMES that were lighted inside a really big statue that looked like the god Moloch. When children were given to Moloch, priests played loud trumpets to not hear the cries and screams of the burning babies. During the battle with Asgore, you can hear music that sounds like very loud trumpets, I mean, Toby Fox uses trumpets a lot on his music because of his Touhou-like music style, but these trumpets are special, they have a ROYAL sound to them, it certainly sounds like the prelude for a sacrifice to a KING. Which king? The KING Moloch. Another interesting fact is how Asgore uses a trident, an object often associated with a demonic figure. Also, check the CORE machine, you can see it looks a bit like a Moloch statue. If you look closely; there are two hands and two horns (and the hands make it look like it's Moloch sitting in its throne). And under this machine there is fire and flames. Inside the CORE, there is an hexagonal symbol everywhere in the floor that resembles Saturn's North Pole and that hexagon is curiously enclosed in a square. One of the symbols of Saturnian worship is a black CUBE (which is used frequently in the Abrahamic religions). Who is Saturn? Samael. Who is Samael? Satan. Who is Satan? THE DEVIL. The hexagon is also a symbol of Metatron (Metatton) which is refferred to as "Metatron's cube".
I also found in the Internet someone mentioning strong similarity to those pillars and the black dot in the sky at the introduction scene when you first fall with the Masonic pillars and the All-Seeing Eye of Saturn.
Gaster's number is 666, and everyone knows that's not God's number, but Satan/Saturn's number instead. Gaster fused with its creation the CORE, but he also created the CORE, so he is GOD, but actually he's El, Elohim, the supreme deity of the Ancient Hebrew pantheon (ISRAEL means ISIS-RA-EL, the three Boss Monsters from the Dreemurr family represent this trinity of gods, forming the Masonic symbol of the triangle, with Saturn's All-Seeing Eye on it) and the CORE is a Moloch statue, Moloch later became known as "BAAL MOLOCH" (LORD MOLOCH, LORD KING) then just became "BAAL" (LORD). Baal was the demonic offspring/brother of El who was also deemed as a god. Gaster fusing with the CORE symbolizes the corruption of El's cult and its fusion with Baal's blood cult becoming the modern Hebrew supreme deity which is Yaldabaoth (the Demiurge, which has nothing to do with the actual God, but tricks us into believing he is). Alphys replaced Gaster... ALPHYS... AL/EL PHYS... El Physician, a scientist of El. Also there are two demons in the Ars Goetia, from The Lesser Key of Solomon (an ancient occultist grimoire that allows you to summon demonic gods from archaic Mesopotamian mythologies) that sound like "Alphys"; Malphas and Halphas, interestingly these spirits control towers, mechanisms and can build artifacts, plan strategies, all things Alphys does in the game. Malphas can tell you the thoughts and plans of the enemies, and Alphys will tell you what Metatton's going to do and how to stop him. Halphas sounds like "HELP" and Alphys offers HELP. Malphas hates sacrifice, and will attempt to trick you if you try to do it, this might be reflected at Alphys being the only character refusing to directly kill you in the Genocide Route and instead leaving some traps on. Malphas color is ORANGE and Halphas favorite scent is Dragon's Blood. What does Alphys look like? An ORANGE DRAGON.
Sans is the NSA, SANS/SA-NS/NS-SA/NS-A/NSA. This is why he seems to be everywhere at the same time, and seems to know everything you do and it's mentioned that there are "others like him". Papyrus represents the society as a whole, the bottom of the pyramid, infantilized, distracted by false promises (like that he will become a part of Undyne's Royal Guard one day). Sans (the NSA) ensures that Papyrus doesn't "misbehave" and toys with him in a seemingly "innocent" way. This is to ensure he follows the plan and doesn't rebel.
Napstablook represents the philosophy of the Egoism, of the writer Max Stirner. Napstablook acts as the esoteric idea of the "SPOOK" (which is why he is a ghost) which is a false idea without actual meaning that we believe it's real and only serves as an obstacle to reach the Enlightenement and the Truth. Laws, Rights, the State, Property... They are all spooks. Napstablook, becomes a kind of annoying obstacle at the Ruins, and we have to go through a battle with no actual purpose to pass. Indeed, the behavior of this character is very nihilistic, which resembles the egoistic philosophy of Max Stirner. The name "Napstablook" could be interpreted as Nap-Stab-Look but that's only an spook, and if you look carefully you can make the word "spook" from it; napStablOOK, shift the "b" and you get a "p"; napStaPlOOK; S P OOK; SPOOK.
Then there's this last character called "SO SORRY", which the creator claims he did put there because someone paid him to do so, but that's not true... he represents sexual pervertion, a sexual abuse satanic ritual. To find it you have to do a kind of ritual at a specific time and date, at the outside of the CORE. It is known of course that the worshipers of EL, known as satanists, perform rituals of sexual abuse and human sacrifice, herein this character is despised by many players that don't even know what he TRULY symbolizes. This character is there to symbolize sexual perversion into dark rituals of black magick which happens in the temples of Saturn, he is "SO SORRY"/"SOSORY"/"SORCERER", a play of words. The fact that the creator mentions "he's there because someone paid him to do so" just symbolizes that his whole existance is linked to money, greed and lust. You can sense during the "SO SORRY" fight an awkward feeling, it's because it emulates the feeling of experimenting severe sexual trauma during a black magick abusive ritual. The experience itself feels weird and traumatic, and it's because it's meant to be so. This creature has a reptiloid tail, so he's a reptilian, and he wears a hat... the hat of Saturn.
We can see more sexual imagery and abuse references in the game, like the homosexual intercourse between the Royal Guards when you enter the Hotlands which looks like a degenerate ritual in which the guards slowly start getting naked and flirting to each other, with the Player giving fuel to this which makes it look like they are inviting YOU to a trio and YOU are being a part of the ritual. All of this at the Hotlands, in front of the CORE which is a Moloch statue. A typical feature of Moloch rituals were sexual activities like orgies, masturbation or rape of the to-be-offered kids. These "Royal Guards" resemble Knights of the Templar Order, which worshipped a goat-like (Asgore?) being called "BAPHOMET". These Knights did ritualistic sodomite orgies to glorify their deity, Baphomet.
The game is constantly playing with symbology... everything is illusions. The Annoying Dog is there to symbolize the thought of an Annoying God, the game's creator thinking. Toby Fox is nothing but a nickname, another symbology you can find in other kinds of media. Gaster is the Player, the creator living its own creation to metaphor that the mistery will live forever and ever, everywhere, even outside of time and space which is represented by the game universe and so he is inside as well, omnipresent and omniscient. That's why his mistery is so compelling and why no one will ever find anything else more about him inside the game. Gaster in Gematria translates to "Enter Hell", "Demonic Images" and "End Times". What you are seeing in the Underworld is nothing but the World of EL. The world that the illuminati ELites want to establish with the New World Order. Then Chara (THE DEVIL) will tell if you want to finish this timeline to move on to the next one. It doesn't matter what you say, the world will be finally destroyed as you were never in the control, but El was. The End Times finally come. The simulation is over, but it can start again, at the price of your SOUL. And the whole cycle starts again, over and over. This is the meaning of the Matrix.