First thing he absorbs is specifically a poison/skeleton dragon of sorts.
I'm also not sure if he was summoned on his own or with his classmates, but he remains a human after being transported.
It's in black and white with no nudity or gore as far as I can remember.
Main character is a generic Isekai protagonist with short black hair in high school. He absorbs the enemy's stats and skills through his hand or by eating them.
It's probably "I’m Capped at Level 1?! Thus Begins My Journey to Become the World’s Strongest Badass!" (Genkai Level 1 kara no Nariagari: Saijaku Level no Ore ga Isekai Saikyou ni Naru made), licensed in English by J-Novel Club (first chapter available as free preview on the site).
While trying to save a girl from four thugs, he and the girl are accidentally summoned to another world along with the thugs. As he is capped at Level 1 he is thrown into a ravine. His only skill is "Corpse Absorb" but he encounters the rotten corpse of a dragon which increases his stats immensely allowing him to survive against the other monsters down there, and meets a catgirl and an elf (IIRC) disguised as a teddy bear who are similarly trapped.
Is this I'm the Only One With Unfavorable Skills, Isekai Summoning Rebellion...?
The student Nakatani Yuto was supposed to be having a regular life, however his whole class was transferred to another world. All the students gained powerful skills and were worshipped as heroes, but for Yuto who got the unfavorable skill [Absorption]...! ?
The protagonist, Nakatani, was a Japanese high school student who was summoned to a kingdom in another world along with the rest of his class. Each of them gained a special skill, but his skill of absorption was considered extremely weak, and so most of the others viewed him as baggage rather than someone useful. (He absorbs skills and powers through his hand.)
He ultimately ends up separating from the other students while they're exploring a dungeon, and meets a demon girl who was imprisoned within the dungeon by the same empire that summoned the students. She explains that her demon race wasn't hostile to humans, and that the humans in this world were the aggressors who attacked and enslaved her race. She wants revenge for that, and invites Nakatani to absorb her power so that he'll be strong enough to help her achieve that. He does so, freeing her from the magic that was imprisoning her in the process, and she subsequently becomes his companion.
Not long after that, while still in the dungeon, they encounter a huge monster referred to as a 'Corpse Dragon,' and after a battle, Nakatani absorbs its power, which grants him the attribute of immortality.
