Super Sentai is a franchise where you have colored heroes fighting evildoers trying to destroy the Earth, killing humans, and other evil stuff. In a lot of cases, the villains are either aliens, or demons, or evil sects, evil robots, or whatever creature wanting to conquer the Earth and destroy humankind.
However, in a lot of sentai series, there are villains who are closely related to the heroes, and even may have had a strong relationship with one of the heroes or even several heroes. As a result, the rivalries between the villains and the heroes they're related with can be even stronger, and it can result that, instead of fighting only to save Earth, the fights have a more personal feeling.
Let's review sentai series which use that concept (and I'm talking about main villains, not merely one shot villains)
Some series have the concept of villains having a close connection to the heroes being central in the conflict between heroes and villains
Liveman is the most classic example, where the most important generals of Volt are none other than the former friends of the three main Liveman; Kemp (Kenji Tsukigata), Mazenda (Rui Senda) and Obler (Gou Omura) are former classmates of the heroes at the science academy; Gou was Jou's good friend, and Yuusuke was Kenji's friend and even had a crush on Rui ; betrayal is a big theme of Liveman and the heroes suffer when they see them kill two of their friends, and then, trying to conquer the Earth and destroy humanity; a lot of episodes deal with how the fight between the Liveman and the Volt is highly driven by personal feelings, and the struggle of those heroes, their inner conflicced feelings, either of desire of to have revenge and defeat the traitors or to forgive and save their former friends (and succeeding in one case); it must be noted that Gou Omura ends up leaving Volt and becomes back Jou's friend; a good example of a case of someone who ended up realizing his foolishness and decided to revive his friendship with his former pal; that said, it's even more interesting to notice that, despite becoming good, Gou still has strong friendship feeling for his former Volt comrades, as seen when he tries to warn Mazenda from Bias's evil plans, and how he's sad seeing her die. Never in a Super Sentai series, the conflcit between heroes and villains was so personal.
2) The second half of Turboranger
While the Boma were just demons, the Nagare Boma Yamimaru and Kirika were going to the same school as the heroes; in Kirika's case, she was even a former childhood friend of Shunsuke, and Riki's friend and fan. starting with Kirika's appearance, the NagareBoma became the main antagonists of Turboranger; again the theme was how to deal with those classmates who became evil; and is that possible to get them to stop fighting a find peace; it's especially true for Kirika who had shown before her good side. In that case, the heroes succeeded, first with Kirika, who, after struggling with her identity during most of her arc, finally found peace between both her human and Boma halves, and as a result, managed to get Yamimaru to stop fighting.
Dairanger had several plots involving deep
connections between heroes and villains. The major ones involve Kou the
sixth ranger, Master Kaku the mentor, and Kujaku, a supporting character : Kou is none other than a
half Gouma, brother of the evil Akomaru and son of Shadam, and the
rivalry between him and Akomaru si a very strong one; however, neither
one know that they are siblings, and Kou never learns that he's Shadam's
son. Doushi Kaku is a former Gouma who wanted peace instead of
conflict, and as such was in conflict with his fellow Gouma, but still kept a connection with the Gouma, being able to talk to the Goumaemperor and having followers among the Gouma. The rivalry between Kujaku (Daigo's lover) and Gara, fueled by feeling of betrayal is also an important plot, and also involved Shishiranger/ Daigo. lastly, the rivalry between Ryou and Jin deserves to be mentioned as well.
A lot of the major main villains of Abaranger had a connection with the heroes; Destruction Messenger Jeanne, Messenger of Darkness Gueilton, and Messenger of Dawn Lije (and later Lijewel) are none other than Asuka/Abareblack's wife Mahoro, his brother in law (and former best friend) Mizuho, and his daughter; while Gueilton only lasts two episodes, the conflict between Abareblack and Jeanne is one of the most important plots of Abaranger, with once again the theme of betrayal involved, even if, in that case, the villains's rivalry was fueled by their feeling of betrayal because they felt betrayed by their husband or friend. While Asuka didn't know that Lije was her daughter, when Mahoro had to go back to the Evolian as Jeanne as a spy, she had to deal with her daughter being the evil Lijewel. Another major plot was the rivalry between Abared and Abarekiller, the latter being the surgeon who had saved the former before. Because of that, Abared had a lot of trouble dealing with Abarekiller's evilness.
The conflict between Magiranger and Wolzard is worthy to be mentioned, since Wolzard was none other than the sibling heroes's dad and Magishine's mentor; it's especially interesting to notice how, despite having forgotten his identity, Wolzard remained an honorable warrior, and subconsciously, wanted his kids to be stronger. Another strong conflict was the one between Lunagel and Hikaru/ Sungel with their former Heavenly Saint comrade turned traitor, Raigel/Meemy.
Rio , the leader of the RijyuKen Akugata, and main villain of the series, was none other than Shafu (the Gekiranger's mentor) former pupil, and Gou/Gekiviolet and Miki's fromer friend and comrade. Besides, Rio had a rivalry with Jan's dad, and that rivalry was revived with the son. As such, the personal component of the antagonism between the Gekijyuken and Rio was even stronger. Moreover, it's important to mention the rivalry between the Seven Kensei (mentor of the Beast Arts school) and the three Kenma (mentors of the Akugata school) who makes also a major arc of Gekiranger, since once again those are former friends who became enemies. Last but not least, there is also a the conflict with Jan and Suugu, a chimera monster who had the Gekisoul of his father, and as such, is the closest being that Jan can cal his dad
Two plots are worthy of mention: the conflict between Marvelous/ Gokaired and Basco, who were both Red Pirates before Basco's betrayal, and Joe/Gokaiblue having to fight Barizog, hos former friend turned into a mindless zombie
Other series are using villains having connection with the heroes, but less developed (but still important)
In Bioman, Red One's father did know Dr Man; besides Dr Man's son became an ally of the Bioman
It may seem strange to have Maskman there, but Igam's strong rivalry with Takeru was more linked to Takeru's relationship with Princess Iyal, Igam's sister, rather than a more personal connection. However, it's still a strong rivalry; and Kiros, who's in love with princess Iyal, also sees Takeru as a rival.
Vyram general Maria was none other than Ryu/ Red Hawk's brainwashed girlfriend; however Ryu only really realizes it late in the series, and it doesn't really fuel Maria's conflict with Ryu through the series, at least until the second half. However, it's still an important plot, especially because of its effect on Red Hawk
Geki's rivalry with his brother Burai/ Dragonranger deserves to be mentioned, despite only lasting about six episodes (bit those episodes are some of the most memorable of Zyuranger)
Carranger had a subplot involving a love story between Red Racer and the female Bowzock Zonnette; the latter's sister also appear as an ally to the Carrangers.
Dr Hinelar, who is the heroes's main antagonist, was none other than Dr Samejima, a former colleague and friend of the heroes's mentor, Dr Kubota
The Gouraigers, who were from a rival school from the Hurricanger, were the heroes's enemies before becoming their allies and friends
Series where the heroes had a personal conflict with the villains, precisely because they personally suffered because of them, and that suffering being an huge plot in the series
The heroes were themselves victims of Mess, having been kidnapped as children by the Alien Hunter, led by Ser Kaura, because of the evil Empire
The five siblings lost their parents (even if their parents ended up having survived) because of Zone
Daimaoh pretty much had a devastating effect on Tsuruhime's family, especially by forcing her father to work for him (even if he was revealed to be a double agent)
The heroes lost their forest and home because of the Balban
Timepink lost her family because of Don Dolnero; and the shady role of Captain Ryuuya must be mentioned as well
The Vaglass have pretty destroyed Red and Yellow Buster's families, and Jin Masato became also a victim as well.