Ash's Infernape was originally Paul's Pokémon as a Chimchar, but after it was released by Paul, it proved to be a very strong Pokémon. It was also one of the main plot lines of the Diamond and Pearl series.
As a Chimchar, Infernape was discovered by Paul whilst trying to escape a pack of Zangoose. When it was trapped on the edge of a cliff, Chimchar used Flame Wheel and defeated all the Zangoose with a single attack. Paul was impressed and decided to catch it.
However, Paul grew disappointed because Chimchar never displayed that kind of power whilst it was training with him. Although Chimchar helped Paul win many battles, including his gym battle with Roark, Paul still considered it weak and when Chimchar defended Ash's Turtwig from a Fire Blast in Glory Blaze, Paul had enough. Deciding that Chimchar was too soft to use the kind of power he'd seen when it battled the Zangoose, Paul released it from his team for good and Ash invited it to join him instead which Chimchar happily accepted.
After joining Ash's team, Chimchar overcame its fear of Zangoose and was used in many battles. Chimchar weakened Maylene's Meditite and Lucario and defeated Fantina's Drifblim and Mismagius as well as Byron's Steelix and Bronzor and Candice's Abomasnow. Chimchar finally evolved into Monferno during Ash's first full battle with Paul after defeating Paul's Ursaring for the third time. Despite evolving , however Monferno was defeated by Electabuzz. which meant that Ash lost the full battle. Later on in the series, after being knocked to the brink of fainting in a battle against Barry's Empoleon, Monferno went berserk with its Blaze ability and unleashed its full power against Team Rocket. While protecting the Pokémon, it evolved into Infernape. Infernape has many powerful attacks at its disposal such as Flame Wheel so it's set to be the powerhouse behind Ash's team. It was later used in a battle against Flint's Infernape, but lost due to its lack of experience. Like Paul's Torterra this was its only defeat since it evolved from Monferno and it has won every other battle it was in.
Infernape learned Flare Blitz in Familiarity Breeds Strategy, knocking out Brock's Sudowoodo. It later faced off against Paul's newly evolved Aggron with Flare Blitz which burns it and proceeded to deliver a Mach Punch defeating the powerful Aggron. It later battled Paul's Ninjask. Infernape used Dig and used its Flare Blitz underground which made the field burst out Infernape's flames which damaged Ninjask and canceled Paul's Drapion's Toxic Spikes effect and mist surrounded the field which made Infernape attack Ninjask by a single Mach Punch. Infernape battled Paul's Electivire in the final battle between Ash and Paul. Electivire used Thunder and slammed its tail to the ground which made the rocks fly towards Infernape. The so-called "Field Thunder" was done because of Infernape weakening the stability of the field when it used "Underground Flare Blitz". After a good battling, Infernape used Mach Punch. However, Electivire's tail caught Infernape's arm, constricting him. Electivire then proceeded to use Thunder for a long period of time. After Electivire let him out, the referee announced Infernape unable to battle, but Electivire prevented him from doing so. Suddenly, Infernape began to glow and activated its Blaze and fired a super-powerful blast of Flamethrower which not even Electivire's Thunder could penetrate. Infernape then attacked with a powerful Flare Blitz. Thanks to the Blaze-powered Flare Blitz, Electivire was burnt, gave way and stumbled to the ground, declared unable to battle, with Ash winning the full battle against his long-time Sinnoh rival Paul and ending their rivalry. In DP190 it helped make the ship hot to heal several Baby Pokémon.
+ indicates this Pokémon used this move recently.* - indicates this Pokémon normally can't use this move.
Infernape has used its ability Blaze a number of 5 times, twice as Chimchar, once as Monferno, and twice as Infernape. It lost control as Chimchar and Monferno, and finally controlled it as Infernape.
Infernape has activated its ability more times than any other Pokémon on Ash's team.
Infernape's Flare Blitz seemed to be stronger than that of Flint's Infernape, as Infernape's Flare Blitz is blue instead of the Flint's Infernape's Flare Blitz, which is red and yellow. Blue flames are hotter than the normal ones.
However, Flint's Infernape was seen using a blue Flare Blitz whilst battling Cynthia's Garchomp.
Infernape is the first Pokémon owned by a rival from Ash, and which was later owned by Ash himself.
All of them are Fire types that evolved from starter Pokémon.
All of them were abandoned by their original trainer because that trainer thought they were weak.
All of them became among Ash's most powerful Pokémon, proving that their original trainers had been wrong about their weakness.
Unlike Charizard and Pignite's original trainers, Paul did not lie or fake a kind farewell to Infernape, but rather straight forwardly demanded it leave.
Another difference is that unlike Charizard and Pignite's original trainers, who had the audacity to ask them to rejoin their team despite the appalling way they treated them, Paul did not attempt to get Infernape to rejoin his team after seeing how powerful it was, thus sparing himself the humiliation of grovelling to the Pokémon that he had abandoned.
This could also be seen as Paul's one and only act of kindness for Infernape, as it means that Paul truly acknowledges Infernape's growth from the weak and timid Chimchar he abandoned.
After evolving into Monferno, it become the first time Ash had two starter Pokemon that evolved from the same region.