About me: Splinter is portrayed as wise, intelligent, and a skilled "elderly martial arts master". He is nearly always calm. Even when angry, he refrains from raising his voice. He is the quintessential calm, all-knowing, and wise master of all martial arts.
Splinter cares for his adopted sons with fierce devotion, rescuing them in very critical moments in the series, such as when Shredder attempted to execute the four on a building, or when Bishop tried to literally tear them apart for science. He is furious when the Foot attempts to slay the Turtles with a robot Splinter and goes all the way to Japan after the four are kidnapped by the Tribunal.
Despite his love for his sons, Splinter is fairly militant with them, especially when they are young and inexperienced. Splinter's main fear is that he and his family will one day be exposed to the outside world, as he is understandably and protective. He disciplines the turtles when they become disobedient or unruly. His punishments include making them do back-flips repeatedly in the second live-action movie, or being sent to the Hashi, a form of punishment in the 2014 film by using chopsticks for balance.
Splinter is not completely cut off from the pleasures of modern culture; he enjoys soap operas. This is more fully displayed in "Fast Forward" where that hobby of his is mentioned several times. It is also mentioned in both the 2007 animated film and the 2012 series that he very much enjoys dessert, particularly Ice pops.