Theorizing has been kept to a minimum in this thread as I prefer to only talk about canonical lore. For those new to VALORANT Lore, I have added a glossary of any lore-specific terms used in the thread.

Viper is Agent 02 of the VALORANT Protocol, meaning she was the second agent to join the organisation, after Agent 01, Brimstone. She is also one of the organisation's founders, alongside Brimstone. Her real name is Sabine, although she dislikes being called by it.

Viper is a chemist from Alaska, USA. She uses an array of self-made poisonous chemical devices to control the battlefield, cripple the enemy's vision and wear them down, both physically and mentally. She thoroughly enjoys playing with her food, employing various mind games and observing their deterioration with great interest.
Viper is a very sadistic person. She thrives on the enemy's suffering and prefers slow, painful methods to kill her prey by her poisons and toxins rather than simply killing them. She also displays multiple signs and symptoms of sociopathy, not caring for the safety of herself or others and showing no remorse for the harm she inflicts.

History/Past
Before becoming the VALORANT agent that she is today, Sabine was a normal chemist who used her research and skills to heal others rather than inflict pain like she does today. She is implied to have worked with or worked for the Kingdom Corporation, and researched Radianite and Radiants as part of her work. However, something significant happened to her that caused her to lose everything, and this event had a toll on her mental health, possibly being the cause of the sadistic and sociopathic nature she possesses in the present. As a result of losing everything, Sabine turned to a path of violence and revenge, using the Radianite technology she had closely been studying in the past to create poisonous chemical devices: gas emitters, chemical launchers and sprayers, all using her own potent toxins and gases. Viper's connection to the Kingdom Corporation and her new found path of revenge made her the perfect candidate for a founder of the VALORANT Protocol. Blaming the Earth 2 attackers for what happened to her in the past, Viper now takes extreme joy in taking part in VALORANT missions, protecting her home and simultaneously observing the suffering and pain of her opponents.

Relationships
Breach - Being one of the VALORANT Protocol's founders, Viper personally hired Breach to join the organisation, although Breach's induction was not a smooth affair. They have a mutual respect for each other, although their relationship isn't much more than two colleagues.
Brimstone - Brimstone, being another founder of the VALORANT Protocol, works closely with Viper in matters regarding the organisation. Although Viper dislikes his moral approach to handling and leading missions, their relationship is definitely more complex than just colleagues. Like Viper, Brimstone also has a connection to Kingdom, so it's possible they knew each other before the VALORANT Protocol as well. In some cases, Brimstone also calls Viper by her real name, Sabine, which suggests they are friends, just with a very complicated relationship. Both know a lot of information they have not revealed to the other agents.
Cypher - Viper is interested in the information Cypher collects about the Earth 2 attackers, and often approaches him regarding it. However, their relationship is strictly professional.
Killjoy - Viper and Killjoy are the VALORANT Protocol's two scientists that we currently know of, and both run tests of some sort while on the battlefield. Viper refers to Killjoy as a "little mouse", implying she is condescending towards Killjoy as a scientist, being older and having more experience in the lab.
Omen - Viper and Omen have definitely crossed paths in the past, and it is heavily implied that they have known each other since Omen was a normal human being. Voice lines also suggest they were close friends in the past, but something (possibly Omen becoming a Radiant) complicated that relationship, as both only consider each other as old friends. Despite this, they do seem to care about each other, even if they choose to hide it.
Reyna - Before the VALORANT Protocol, Viper experimented on a little girl with a connection to Reyna (possibly Reyna's little sister), whether it was with good intentions or bad intentions, we don't know at the moment. However, the experiment was likely to have resulted in harm to or the death of the girl, and this may have caused animosity between Viper and Reyna. Viper notes that Reyna seems to be after her, although Reyna does not seem to possess extreme hate towards Viper, possibly because she does not know Viper's true identity.
Sage - Viper and Sage have crossed paths in the past, and whatever happened, was not a smooth affair. Viper says that Sage "failed" her in the past, suggesting they worked together on something, but Sage was unable to do what she was expected to. Whatever happened, it has definitely stuck with Viper, and she is very bitter and resentful towards Sage about it.

Glossary
VALORANT Protocol - An organisation or protocol that brings agents from around the world to fight and defend Earth from attackers and radianite based events
The Kingdom Corporation - A mega corporation that harnesses the energy from radianite to benefit humanity. Their radianite power program accounts for more than three quarters of the world's energy production. Their headquarters is located in Tokyo, Japan at the map Split and extract radianite from landmarks and cities around the world, including the city of Bind in Morocco, and the monasteries located in Thimphu, Bhutan, near the site Haven. They are implied to have a connection to the founding and administration of the VALORANT Protocol, possibly being the main people behind it's foundation.
Radianite - An element that was discovered after First Light, the cataclysmic event that impacted Earth years ago. Radianite is very powerful, and the Kingdom Corporation have found a way to produce power and energy out of it. It is also capable of turning humans into radiants if they come into contact with it.
Radiant - A human or individual with supernatural powers as a result of contact with Radianite. Some of the VALORANT agents are Radiants, including Astra, Jett, Omen, Phoenix, Reyna, Sage, Skye and Yoru.
Radianite technology - Advanced technology powered by Radianite. The non-radiant agents use this type of technology in combat to help them against the powers Radiants possess.
Earth 2 - The other Earth from another reality. Both Earth 1 (our Earth) and Earth 2 are in conflict over Radianite, as both Earths need Radianite to power themselves. Earth 2 sends VALORANT strike teams to plant spikes at Kingdom worksites to steal radianite, whereas our Earth's VALORANT Protocol sends strike teams of agents to stop the detonation of the spike and protect our radianite.

If there are any other members of the VALORANT Lore Community reading this, please let me know if I've missed anything and feel free to theorize about Viper's lore in the replies! As for anyone who isn't familiar with VALORANT Lore, feel free to ask questions and I or someone else from the Lore community will hopefully answer! I will make more Lore posts about the other agents as well so keep an eye out if you're interested.