Prior to the start of the series, Red owned Poli as a Poliwag. While swimming one day Red began to drown and Poli was unable to do anything but watch. Due to being unable to help Red in his current state he evolved into Poliwhirl and saved him. Poli first appeared in A Glimpse of the Glow, helping to capture a Nidorino for Red. Poli later battled against a wildMew, but was knocked out in a hit.
In Onix Is On!, Poli, Saur, and Pika helped Red in his Gym battle to win the Boulder Badge. Poli fought in Red's first battle against a Trainer's Graveler and was able to win by using one-hit knockouts and complimentary moves, despite starting off with low health.
As a Poliwhirl
In Danger: High Voltorb, Poli was the only member of Red's team to infiltrate the S.S. Anne. Immediately upon entering the two of them end up battling a Voltorb, with Poli ending up paralyzed. Due to Poli being paralyzed and unable to help out Red is forced to dodge the powerful Thunder Punch attacks from Lt. Surge's Electabuzz. Right when it seems the two of them are trapped Red reveals their plan, which had Poli secretly drip a trail of water on the floor leading to Lt. Surge and Electabuzz. At Red's command Poli uses Ice Beam to freeze the legs of Lt. Surge and Electabuzz, and with the smoke of a launched Poké Ball the two escape safely. While trying to find the exit Poli is attacked by a Magneton and captured by Lt. Surge, who throws him off the ship. Red attempts to rescue him, but is electrocuted and throw into the ocean as well. As Red begins to recall a similar situation he is saved by Poli, who evolved into a Poliwrath. With his new found strength Poli easily out powers Electabuzz with his new Seismic Toss. In The Kindest Tentacruel, it is revealed that Poli was able to evolve without a Water Stone due to the Vermilion Harbor having traces of the four evolutionary stones speculated to be at the bottom of Vermilion Harbor.
In A Tale of Ninetales, Poli is accidentally traded to Blue alongside Saur and Pika. During the three days that Blue has them he trains them vigorously. They later traded back, while keeping the serious attitude that Blue installed in them.
In Meanwhile…Vileplume!, Poli battled in Red's Gym battle against Erika in his fight to save Eevee. Poli goes up against her Bellsprout, with Red being confident that it will be an easy victory. Poli attacks with Double Slap but is stopped on the last hit and counter attacked with Bellsprout's Mimic, knocking Poli out.
In Golly, Golem!, Poli is picked as Red's first Pokémon in his battle against Giovanni. Red send out Poli to attack, but in the short amount of time that he does so, Giovanni successfully releases his Cloyster and freezes Poli.
During the Pokémon League, Poli battled against a Trainer's Kabutops, advancing Red to the semi-finals. In A Charizard…and a Champion, Poli and Pika helped Saur defeat Blue's Charizard in a brilliant strategy by having Poli's water vapor and Pika's electric energy create a thundercloud that struck a vine shot and wrapped around it by Saur.
In Punching Poliwrath, Poli is shown in a flashback fighting against Bruno's Hitmonchan. Poli was easily able to block all of Hitmonchan's attacks and strike back with Double Slap, but was taken out when Hitmonchan attacked with Thunder Punch and Fire Punch. Poli battled in the rematch against Bruno in Clefabulous Clefable, where he is battled against Machamp. Machamp quickly overpowers him after his belt is removed and holds Poli down. Vee manages to use the water Poli left to use Acid Armor to surprise attack Machamp, allowing him to defeat it.
Being Red's first Pokémon, Poli has a strong relationship with his Trainer. Due to all the time they have spent together they can formulate plans without the use of words, which Red says they used to use to escape from bullies. Poli cares deeply for Red, saving him from drowning by evolving on two separate occasions. Poli has good battling skills, even though he often loses the battles he participates in.