Heroic Spirit of assassinations. Unlike the other classes, the Assassin class is only ever supposed to be one particular Heroic Spirit.
The Assassin in the story was a samurai guarding Ryuudou Temple’s gate.
Sasaki Kojirou was summoned into this class solely because his summoner violated the rules.
Strictly speaking, he is a wraith, not a Heroic Spirit. It seems that in the past, he was a master of martial arts with some connection to Ryuudou Temple.
His true name is unknown. Or rather, it would be more accurate to say he didn’t have something like a true name in the first place.
Perhaps because he never had the possibility of serving a lord, he selflessly devoted himself to the sword, and eventually attained a technique with the same level of mystery as a Servant’s Noble Phantasm.
In terms of pure swordsmanship, he was the strongest Servant in the Fifth Holy Grail War.
Because he was summoned using Ryuudou Temple itself as a catalyst, he cannot leave the temple gates.
…Hmm? Doesn’t this feel a bit like a haunting?

Fate/side material: Fate Encyclopedia

He is an assassin that melts into the shadows. Julius is his Master.
His true name is Li Shuwen.
A person who lived from 1864 to 1934.
Li was a prominent Chinese martial artist who was feared for his prowess in battle, “Li Shuwen needs no second strike, as long as he lands one blow it will more than suffice.”
He became known as “Fierce Fist Li” for casually ending the lives of those he defeated in martial art matches.
In the game, he stands out from the other Assassin class Heroic Spirits because of his unique skill “Presence Concealment,” which he uses to impede the protagonist as an invisible assassin.
While cold-hearted and able to kill without reserve, he is not someone who takes pleasure in killing: “When I encounter another warrior, one of us will die. A fight is not something that can be taken lightly; it is something upon which both parties have staked their beliefs. Therefore it is not something that should be approached in a casual way. Also, after a fight has begun, it must always result in the death of one combatant.”

This is the principle which governs the Assassin in Fate/EXTRA.
He is extremely logical and fair, and it would seem that this is what he believes a fight to be.
His Master, Julius, also makes his living by taking the lives of others. Their compatibility is good.
Julius was originally a contract killer, and Assassin was originally a professional martial artist, so their opinions clash on a fundamental level.
Julius kills due to his profession,
while Assassin enjoys a fight to the death because he is a warrior.
Although Assassin is not a maniacal killer, he feels joy once a fight starts.
Assassin would heartily laugh and say that without such an attitude it would be impossible to reach the height of martial arts, to which Julius replied seriously: “……so that‘s how it is.”

Li Shuwen belongs to the Assassin class, however only when using a lance can his true ability be drawn out to the fullest.
He is the founder of a branch of Bājíquán named Eight Extremities Gate [Bajimen], and he excelled in the use of spears to the point of being nicknamed God Spear Li.
The Six Harmony Great Spear [Liuhe Daqiang] that he used is the basic weapon of Eight Extremities Gate, and to speak strongly, it can be said that the unarmed techniques of the Eight Extremities Gate are nothing but the preliminary step to learning the techniques of this great spear.
Simply (but not purely) Li desired nothing more than to increase his own strength. In his later years he came to see the power of his values and way of life as his strength, rather than violence.
He acknowledges that he is a closer to being a professional killer than a martial artist, but this is neither a self-depreciation nor a boast.
Given that he was good at learning, good at fighting and good at killing in his previous life, he really does not carry any regrets or grudges.
Since he was summoned as a Servant, he swings his wicked fists as the concealed weapons of his Master Julius without hesitation.
For the time being he has taken up the role of Assassin for the sake of compatibility with Julius, though originally his Servant class was supposed to be Lancer. As Lancer he would have probably appeared as a grizzled master, the figure of his later years.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA

A mysterious swordsman wearing a large coat.

Fate/type Redline: Fate/type Redline Glossary