These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Federico the Duke of Vallenberg: To try and take over the Kingdom of Benglait, he first created out of nowhere the terrorist organization Grey Ravens to provoke a civil war. He then married his ticket to the throne, Altea, and faked his death to avoid being identified as the leader of the Grey Ravens. Next, after Benglait became a republic without him being able to take over, he launched a series of terrorist attacks that nearly killed even Altea; after that bit him back in the ass as Diabolik and Ginko dismantled a branch of his organization, he launched other attacks, including a willingly failed one against Altea to justify both his faking his death and then his sudden return as motivated by his love for her and the desire to protect her while still trying to cause that civil war.
There is also Mr Logan: The CEO of a pharmaceutical industry, Logan purchased a Synthetic Plague and its vaccine. His ultimate plan was to release the first into the civilian population in order to wait for the panic and then 'discover' the vaccine, all in the name of money. Logan stands in stark contrast to Diabolik and his more honorable methods of theft. He is willing to kill countless civilians with his manufactured plague, just to line his pockets and achieve fame from it.
Ensemble Dark Horse: Filippo, Gastone and Elietta, the titular characters from the story "Crumbs for Unknown Persons", proved so popular they returned with a major role in "The Heist of the Year 2,000".
In the story The Sweet Death, Diabolik framed two people for an attempted murder they committed and the murder he committed while masked as them. The kicker is, the murder victim asked him to do this: when the two framed people, his wife and her lover, tried to kill him with an 'accident', they left him so crippled he was in perpetual pain and unable to do anything more than blink, so, when he accidentally encountered Diabolik, who was stealing some jewels from him, he blinked a request in Morse code to kill and avenge him, and Diabolik decided to help by killing him and 'confessing' the attempted murder on camera while wearing the lover's mask and with Eva present masked as the wife. The story ends with Altea finding evidence they couldn't have committed the murder and destroying it, as she knew they had crippled the victim but couldn't prove it.
Harsher in Hindsight: In the story Nel Tunnel della Pazzia (Inside the Tunnel of Madness) Elisabeth Gay claimed her hate for Diabolik was born not from Diabolik's driving her to madness but by him choosing Eva as accomplice and partner instead of herself. Her words become harsher after reading Nei Sotterranei di Clerville (Inside Clerville's Underground), that has a long flashback sequence of the times when Diabolik's true face was still unknown and Elisabeth his lover and Unwitting Pawn: the sequence shows that Diabolik did consider her as a possible future partner and actually tested her, with her proving her abilities but accidentally betraying his trust, leading Diabolik to look somewhere else for possible accomplices.
Hilarious in Hindsight: Eva is a very good singer, and used to be a professional singer before marrying. Then in 2012 Italian singer Alessandra Amoroso showed up at a contest dressed in black and with the same iconic hairstyle as Eva...
Memetic Badass: In Italy everyone knows he can take the looks of everyone thanks to his masks (universally known in Italy as "Diabolik's masks").
Lampshaded by the two-parter "A Cursed Island"-"Escape from the Island", where we find out that the natives of the island of Bo-Tang (who provide him with the resin he needs to make his masks permanent) call him Ri-Mau, "The Man with Many Faces and Many Souls" and consider him an emissary of their god precisely because of his masks.