SPOTTED: Tattoos

Khaotung, playing Aye, has some writing tattooed along his pelvis on the left side as well as on his right thigh - which are present on his character

Win has angel wings on his back and a triple-banded geometric tattoo around his right bicep.
Dean has a compass tattoo above his right hip.

Win has angel wings on his back, a triple-banded geometric tattoo around his right bicep, a large bicep tattoo on his left arm, and a valknut on the inside of his right forearm. Also, a reported one below his swim trunks.

Waan has 3 lines of Chinese(?) writing on his upper spine.

Ram has a dreamcatcher tattoo on the left side of his neck, an illuminati symbol on his left pec, a 4-pointed star between his shoulder blades, a geometric and line design along his left side, a geometric tattoo above his left hip, a geometric tattoo with a wolf just below his elbow on his right arm, and a tribal-inspired sea turtle tattoo on his left bicep.

This one actually uses Singto's real-life tattoo on his character Maze: "Save Me From Myself" on his inner bicep.
Singto talked about what inspired this tattoo in this interview.

Mangkorn has a dragon tattoo on the outside of his upper right arm.

Gene can be spotted with a panel illustration tattoo on his left side. Netizens report that it is Up's real tattoo of a family portrait.

Black and White have 4 horizonal bars on their left biceps.
Yok has 3 flying birds on his right shoulder, a line around his right forearm, and a large 母 (mu; mother) along his right side above his ribs.

Kamol has a tiger tattoo that takes up his entire back.

Episode 6: FL has a full back tattoo

Earth (playing as Phupha) has a chevron shade tattoo along his right side, a treble clef-like spiral on his left shoulder blade, and writing on the inside of his right bicep.

Forth has a tattoo along the left side of his neck.

Character: Yoon Ji Woo
All characters that are a part of the organization have the organization's symbol tattooed on the left side of their chest.

Character: Park Jae Eon has a butterfly tattoo

Gu Hye Young has a dragon tattoo along her entire left arm

Character: Mam

Porsche has a phoenix tattoo on his right shoulder blade.
Pete has a tattoo along his pelvis on the left side that says "no legacy is so rich as honesty" in English.

Character: Thian

Character: Daniel Wong and several supporting/reoccurring characters

Several supporting and reoccurring characters have tattoos.

Park Jong Du is fully adorned in tattoos from head to toe.

Meen has a crescent moon tattoo on his right shoulder, which can be seen frequently on his character Ai. The actor also has words tattooed across his ribcage under both his arms, which can sometimes be briefly spotted in the drama as well.

Nine has a tattoo along his left side that says "Nine. IX."

Nine has a tattoo along his left side that says "Nine. IX."
Tee(?) has a tattoo along his right side.

Jae Young has a thin geometric solar system-esque tattoo on his inner forearm in episode 7.  Is it a temp or a sketch? Or did they forget to put it on in episode 4 when we see the inner arm again? Only the webtoon readers might know.

Jang Deok Soo has a snake tattoo on his face and neck.

Film's real tattoos are shown on his character Thee: his family name "Aungvanich" along his right side, writing across his left pec, a sketch of two figures on the outside of his left hand, writing on the inside of his left wrist, and a drawing of a family on his left bicep.

Billy's character Nott is an actor and has a fake tattoo to play one of his characters. The temp reads "Do It From Love Not For Love" across his chest.

Jin-Young has a full sleeve on his right arm that means warrior and is composed of bullets, geometric shapes, and an oni (?) mask, reaching all the way to the center of his chest and back.

Jong-Woo has a sternum tattoo (resembles a dreamcatcher).

Han-Bin has "Time Will Tell" in English below his left clavicle and an image below his ribs on his right side.

Gyutae has an assortment of tattoos up and down his legs, arms and on his back.

Son Oh-Kong had chain tattoos around his arms, legs, and neck in the first episode as a symbol of his imprisonment sentence.

Jin Ho Gae has a temporary tattoo of two crossed swords on the left side of his neck during one episode.

Samura Takatora has tattoos on his face and arms.

Ake has a large lantern tattoo on his right bicep.

Gav has a crossed out "BLESSED" under his left collarbone and a rose with a stem made out of cursive writing on the outside of his right wrist.

Son Myeong Oh has the latin phrase "memento mori" (remember you must die) on his neck.

Most of the contestants have tattoos of various types and locations.

At the end of the film, Mora gets an anatomical heart(?) tattoo on top of her heart.

Palm gets a Chinese tattoo on his upper spine.

Earth (playing as Jim) has a chevron shade tattoo along his right side, a treble clef-like spiral on his left shoulder blade, and writing on the inside of his right bicep.

Pha has an anchor tattoo on his leg.

Silvy, the actress who plays Molly) has multiple tattoos on all her limbs, her chest, and her back - which is shown on her character.

Cir has a two lines of writing on his right side.

Jung Shi Hyun has a full back tattoo

Hua Yutang has a star and moon tattoo on the nape of her neck.

Mark has angel wings surrounding a quote in between his shoulderblades.

Khana has roman numerals down his left side.

Anthika, the leader of a cult, has various tattoos seemingly all over her body - including, but not limited to, a crescent moon.

The male lead has a large tattoo across his back.

Ken has a tribal style tattoo on his right bicep and a scene tattoo covering his back

Prang has tattoos on her arm, one of which was "crybaby" in English on her left arm. 

ML has a tattoo at the base of his neck, in between his shoulders.

IG has a tattoo on his chest.

ML has a thin cross-like tattoo on the inside of his left arm.

Tommy, who plays Nikko, has words all along his left arm and hand - which is seen on his character.

Ford has a figure 8 made of words in between his shoulder blades.
Raver has roman numerals around his left bicep.

Male Lead Ajin has a circular tattoo with flames on his right bicep, a 3 pointed figure on his right shoulder, and eye on the right side of his chest, a snake on his left arm/shoulder/chest, and several more.

Danno has a large ouroboros tattooed across his upper back

Silvy, the actress who plays Maya has an assortment of tattoos on both arms, on her chest below her collarbones, on her right thigh, and in the center of her back - which is shown on her character.
Nawin has wings on his shoulderblades and "Yan" (it was a misspelling of "Charn") written in Thai down his right side.
The senator has a tattoo on his left bicep.

One of the male leads has a large tattoo across his chest - a colored heart, angel wings, and roses with the word "Forever" in English in the center.
Tui has a tattoo on the right side of his chest.

Female lead has a dragon tattoo on her back

One of the male leads has tattoos on his biceps and thighs.

The male lead has his whole torso covered in tattoos

The female lead has several tattoos along her arms

The male lead has a military tattoo along his spine that says "Honor. Destiny. Glory." in between military gear and a praying hands tattoo on the lower righthand side of his torso.

Khaotung, playing Ray, has some writing tattooed along his pelvis on the left side as well as on his right thigh - which are present on his character.

Nick has a stingray tattooed on his right side, a bull head tattoo on his left shoulder, and writing on his left ribcage.

Fai has a written tattoo on his left shoulder blade and another tattoo on his left hip.

The actor Meen has a crescent moon on his right shoulder, "angel" written in English on his left shoulder, writing under his right arm, and a sword and vine and along his right side, which are present on his character.

Ali has a Christian prayer in English written on his right side.

Bom has a tattoo on the inside of his right arm

Sook Hee has a tattoo of a cross and eye on the left side of her chest.

Kiriko has a tattoo on the inside of her right wrist.

Kim has "XXVII" written on the right side of his chest, a tattoo to the side of his left hip, an anchor on his right forearm, a lion on his right forearm, an cross on his right forearm, a fish on his right arm, two lines of writing on the inside of his left bicep, and a triangular gradient tattoo in the middle of his back.

Chen Yi has a Stacked lion and lion cub tattoo, with the Lion King Simba drawing, in between his shoulder blades.
Chen Dong Yang has a lion and rose tattoo on the right side of his chest.

In episode 2, Gong Tae Sung has a temporary tattoo of "Be a King" in English on the right side of his chest for a photoshoot.

Zhang Ruo Ning has a tattoo on the outside of her left bicep.

Pavel, playing Babe, has "Full Savage" written in English on the left side of his chest.

Alan has tattoos on his left bicep. 

Shengnan has various small tattoos all over both her forearms.

Mar has has a butterfly tattoo on the back of her left arm and a sunflower tattoo on her right arm.

Nana as a smiley face on her left wrist and writing on the inside of her left bicep.

Mhok has a banded tattoo interrupted by a valknut symbol around his right bicep.

Jeongho/Hokeep has a tattoo on the right side and back of his neck writing down the left side of his neck, all over both arms, and on his left hand.

Hyeon has a continuous tree-like tattoo on the right side of his neck, an anchor on his left thigh, and cross on his right calf.

Chan Gyu has several tattoos on his left forearm and one on his left shoulder.

Jung Hyun has writing tattooed on the left side of his chest below his collarbone.

Shin has a tattoo on his right pec.

Kim Ga On has a bird tattoo on his left shoulder.