はい

ええN5sense 1
はい is the neutral, standard affirmative used in most situations, while ええ is a softer, slightly more casual or conversational way to say 'yes'.
はっ
はい is the neutral, everyday polite 'yes', while はっ sounds sharper and more formal, as in military or service settings.
ははあ
ははあ as an acknowledgment has an old-fashioned, deferential tone toward a superior, whereas はい is the neutral, everyday word for 'yes'.
応おう
はい is the standard, polite way to say yes in everyday conversation, while 応 is a more old-fashioned, clipped interjection used in contexts like roll calls or dramatic exchanges.

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ヤー
はい is the standard polite word for yes, while ヤー is a casual, playful borrowing from German used jokingly.
しかり
はい is the standard modern word for 'yes', whereas 然り is an old-fashioned or formal interjection with the same meaning.

Opposites

いいえN5sense 1
はい means 'yes' while いいえ is its direct polite opposite, meaning 'no'.
いやいや
はい is the standard affirmative response, directly opposite the emphatic denial いやいや.

うんplainN4sense 1
はい is the polite, formal way to say 'yes', while うん is the casual, informal equivalent used with close friends or family.
えplain
はい is the standard polite way to say 'yes', while え is a casual, sometimes blunt affirmative used among friends or in surprise.
はあplainN3
はい is the standard, polite way to say 'yes', while はあ is a more casual, less formal affirmative that can also carry hesitation or mild surprise.

はいUsage88%
Standard kana spelling for all senses.
ハイUsage10%
はーいUsage1%
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Casual, drawn-out variant often used in friendly or informal replies.
ハーイUsage1%

The origin is uncertain, but it may derive from the Edo-period polite expression はあ, used to show agreement or attention.