PLUTO is a character and optional enemy in OMORI. He serves as a transportation system that OMORI and friends can use to travel to various worlds in HEADSPACE. He appears to be a "giant statue" of rock representing the real-life planet Pluto but with buff arms and legs. He's described as intimidating. 

PLUTO is charismatic and often loud, but also can display a gentle passionate side when discussing his dreams and aspirations. He takes great gratification when showing off his strength, indicated by his favorite tagline: “When I flex! I feel my best!” PLUTO is defined by his unbreakable determination, characterized by a innate desire for freedom and an ability to forge his own path in life, which is why he escapes CAPT. SPACEBOY's planetarium. However, he also shows a desire to impress and help others, leading him into a contract with MR. JAWSUM, and becomes despondent when he proves unable to carry out his duty. [description pulled from the OMORI Wiki]

Consider a deep, boisterous voice. When he's sentimental and emotional, don't be afraid to be silly. Release your inner Shakespearean. Or, even better, give us your best All-Might (My Hero Academia) impression. Low, mighty pitch.