I've been around death from the get-go. You know what I see most? Lies. 'He's in a better place. At least they're together now.' You all lie to yourselves, Dean, 'cause like you said, deep down, you're all scared. Stop lying to yourself, Dean.
Following the car crash in 1.22 Devil's Trap, Dean lies critically injured in the hospital. He is so close to death that his spirit leaves his body. He sees a reaper taking other souls, and tries to stop it, especially when it goes to his room to take him. The reaper departs, but reappears as Tessa, who is supposedly a sympathetic young woman in a coma. She tries to convince Dean that dying is beyond his control, and that it won't be so bad. After Dean realizes who she actually is, they have a frank conversation in which she explains to Dean that vengeful spirits are ghosts who refuse to cross over and eventually go mad. Dean is about to decide whether to crossover when Azazel intervenes, possessing Tessa and restoring Dean to life as part of his deal with John Winchester.
When the reaper in a small town disappears and people stop dying, Tessa turns up to continue the work. Dean does not recall their first encounter, so she restores his memory. Dean and Sam tell her they suspect demons have kidnapped the reaper and ask her to delay her work so they can try and find out where the demons are from Cole Griffith. Tessa agrees on a temporary halt to the reaping, but insists on taking Cole as soon as the situation is resolved. Tessa is then kidnapped by Alastair, who plans to kill both the reapers in order to break one of the 66 Seals. Sam and Dean manage to free her. She then helps Cole Griffith cross over. She also warns Dean to stop lying to himself, that whatever the angels have planned, he knows that there are bad things ahead.
Dean goes to Dr. Robert, who arranges for him to have an out of body experience by stopping his heart. Dean, in spirit form, recites a chant to summon Tessa. She arrives, shocked at the steps Dean has taken. Dean asks her to call Death. She will not do so, but Death arrives on his own and makes a wager with Dean: Dean must act as Death for 24 hours.
Dean gets Death's ring from its hiding place and puts it on. He is transported to Tessa, who has been assigned to escort him for the day. Dean is to kill the individuals on Tessa's list, and she will reap the souls. Tessa tells Dean that the newly dead will often have questions, They first encounter a robber who is shot by a clerk. Dean allows the man to suffer before killing him, and Tessa tells him to hurry up. Next, they encounter a man eating pizza who has a heart attack. Dean touches him, and the man asks what it all meant. Dean responds with a quip, and Tessa tells the soul that Dean's new, and takes him on his way. Tessa then brings Dean to a hospital, and instructs him to kill a twelve year old girl with a heart condition. Dean refuses.
The girl miraculously heals, and the staff that were taking care of her are dismissed for the evening. Later, one of the nurses is brought in from a car wreck, and Dean must kill her. Tessa tells the nurse's spirit that she was meant to live for decades, and have children and grandchildren, but that Dean screwed up. Tessa takes the soul, and when she returns she tries to convince Dean to kill the little girl, insisting that tragedy will always follow her, and that more will die. Dean asks Tessa if she knew what would happen when he refused. She replies no, but that she knew Dean "knocked over a domino." Dean then decides to stop the husband of the nurse that died from dying in a car wreck of his own, and in so doing loses his bet with Death. He returns to the hospital, and agrees to kill the little girl.
After Dean learns from the angelFlagstaff that Tessa is working as a part of Castiel's army, he heads to the hospital where the angels are working minor miracles and finds her. When Dean notices that Tessa has carved an Enochian symbol in her body, he realizes she plans to become a suicide bomber. Just as Tessa is about to overpower him with her strength, Dean is able to bind her with a pair of handcuffs etched with binding sigils.
During Dean's interrogation of her, she reveals that, as a reaper, she has been hearing the cries of the helpless souls that are trapped in the Veil and unable to reach Heaven due to Metatron's actions. She was tormented by the fact that she could not do her job and help them move on to the afterlife, and she eventually contemplated ending her own life with an angel blade, but was ultimately too afraid to go through with it. Castiel gave her something worth living for, toughing through Metatron's forces and reclaiming Heaven. She tells Dean that there are many more like her out there, preparing to do what is necessary. Dean then brandishes the First Blade, much to the surprise of Tessa, telling her he did what he had to do, only for her to reply, "welcome to the club." Tessa then grabs Dean and impales herself on the First Blade while it's still in Dean's hand. Her last words to Dean are "Thank you."
It was later revealed that Tessa, along with a small number of Gadreel's recruits, were brainwashed by Metatron into using themselves as suicide bombers to manipulate Castiel's followers.
Death as illustrated by Chris Bachalo in Death: The Time of Your Life by Neil Gaiman.
Tessa is a shortened form of the given name Theresa. The name is thought to be derived from the Greek verb θήρίζεἰν (therizein), meaning to harvest. The function of Tessa is to harvest the souls of the dead.