Sakamoto is undoubtedly a protagonist but is shin another protagonist or just a deuteragonist
"Dual protagonists are characters who are both the central actors in a story, work toward a shared or similar goal, and take up approximately the same amount of screen time." a simple example is woody and buzz from toy story, 21 jump street, bucky and sam from the falcon and the winter solider and supernatural(not really sure about this i only saw a couple seasons and barely remeber anything)
while "the deuteragonist or secondary main character is the second most important character of a narrative, after the protagonist." an example of this is batman and robin, goku and vegeta and obi wan and anakin
hmm while shin age and power level makes him more suited as a sidekick but i can see him become an equal protagonist to sakamoto as time goes one same how robin becomes nightwing and becomes equal to batman or how anakin becomes obi-wan's equal in revenge of the sith although evil
"Dual protagonists are characters who are both the central actors in a story, work toward a shared or similar goal, and take up approximately the same amount of screen time." a simple example is woody and buzz from toy story, 21 jump street, bucky and sam from the falcon and the winter solider and supernatural(not really sure about this i only saw a couple seasons and barely remeber anything)
while "the deuteragonist or secondary main character is the second most important character of a narrative, after the protagonist." an example of this is batman and robin, goku and vegeta and obi wan and anakin
hmm while shin age and power level makes him more suited as a sidekick but i can see him become an equal protagonist to sakamoto as time goes one same how robin becomes nightwing and becomes equal to batman or how anakin becomes obi-wan's equal in revenge of the sith although evil