He’s a left French politician, several times candidate for presidency but never won.
He is seen as Fidel Castro boot licker by boomers (he is not), extreme left by right and extreme right (he is not).
He is a great talker but also a bit of an angry man which makes him unlikable to others.
Now is party is named “La France insoumise” (LFI) and they usually have very good political agendas, like at the last presidential it was something along the lines:
create the 6th republic based on true direct democracy
constitution is written by the French population for the French population
give back power to the population
once done, quit unless asked to stay president.
But because of his image among boomers and the right propaganda against him, he never won and his party suffer from his image.
The best thing would be to have his and LFI ideas without him. They need to find another leader for their image as Mélanchon is a liability for the left to access power.