-chan (ちゃん) and -kun (くん) are endings added to names. ちゃん is affectionate (small children, close friends), くん is mostly for boys and younger men, and さん is the polite default for everyone.
A character has called her classmate 「たなかくん」 for eleven episodes. In episode twelve she says 「けんた」. Nothing else in the scene changed — and that is the scene.
The ladder
さま — formal to the point of reverence: customers, gods, addresses on envelopes. さん — the everyday default, and the one you want. くん — for boys and younger men, also used downward at work regardless of gender. ちゃん — affectionate: small children, close friends, pets, and anyone you find endearing. Then the bottom rung, which isn't a suffix at all: 呼び捨て (yobisute), the bare name.
Why the bare name is the loud one
In English, dropping a title is nothing; “Mr. Tanaka” to “Kenta” is just informality. In Japanese it announces a relationship. Family use it, couples use it, very old friends use it, and a せんぱい often uses it with a こうはい — but a stranger using it sounds either extremely rude or oddly intimate, with no third option. That's why anime spends whole episodes on it: the suffix does the confessing.
ふじくん。
Fuji-kun.
…ふじ？
...just Fuji?
まだ はやいです。
Too soon.
What should you actually use?
Surname + さん, always, until the other person invites you closer — and they will, either by saying so (「なまえで よんでください」 — call me by my first name) or simply by switching first and waiting for you to follow. Never attach anything to your own name, and never call yourself ちゃん unless you are being funny on purpose. One more trap: くん and ちゃん sound sweet, but used upward — to a teacher, a client, a せんぱい — they sound condescending.