This one's hard for translators - you want to stay true to the original, but can't exactly have the kid call the hero "Big brother" despite the fact that they've clearly never met before. "Oniisan" and "oneesan" and their variants are often used by children for older non-relatives that are not too far apart in age from the speaker (expect the kid Victim of the Week to address the hero this way throughout the episode, for example.) If the addressee is a generation older, "ojisan" or "obasan" (meaning "uncle" and "aunt", respectively) may be used instead and if they are two or more generations older, "ojiisan" or "obaasan" (meaning "grandfather" and "grandmother", respecitvely) may be used as well. It is also not uncommon for sibling terminology to be used for non-relatives.
On the other hand, younger siblings are pretty much universally addressed by their given name. Conversely, whereas calling your older siblings by their name is normal in English, it's a serious breach of manners in Japanese when not attached with a familial term, on par with Calling Parents by Their Name, and definitely marks a distance between the two siblings if used at all.
This is Serious Business, by the way while calling older siblings by "brother" or "sister" is considered old-timey in English, it's part of modern etiquette in Japanese. depending on how he regards Kenta, with the most common way being a simple "oniisan". So the speaker may call his older brother (say, Kenta) "oniisan", "oniichan", "Kenta-niisan", etc. These are used when the speaker is describing someone else's older sibling (see above) as well as when he is addressing his own older sibling. These are normally found in conjunction with an honorific -san and -chan being among the most common for all four, although -sama is sometimes used for respected older siblings, while "baby-talk" equivalents such as -tan, -tama and -chama are limited for very young children. Additionally, ani and ane can take the honorific prefix o- in place of the initial a (and simultaneously double the i / e sound), which indicates further respect.