Та, кто казалась самой доброй, сделала неожиданное признание.

Та is the feminine singular form of тот and here it means the one or she who. In this sentence, та, кто... is the subject of the main clause.

So the structure is:

Та, кто казалась самой доброй = the one who seemed the kindest

Because the person is female, Russian uses the feminine form та.

Та, кто... is a very common way to say the one who... when talking about a person.

• кто is the usual relative word for people in this kind of structure.
• которая is also possible in some contexts, but it feels a bit more specific or heavier stylistically.

So та, кто... is a natural, idiomatic way to refer to a person: that woman / the one who...

Because кто казалась самой доброй is a relative clause inserted inside the main sentence.

The basic frame is:

Та ... сделала неожиданное признание.

The middle part tells us which та we mean, so it is set off with commas:

Та, кто казалась самой доброй, сделала...

This is very normal Russian punctuation for this kind of clause.

Казалась is past tense, feminine singular, because it refers to a female person.

Even though кто itself does not have a feminine form, the verb can agree with the real person being described. Since the antecedent is та, the feminine form казалась is natural here.

So:

• та → female
• казалась → feminine singular

Because the verb казаться usually takes a predicate in the instrumental case.

So:

• добрая = nominative
• доброй = instrumental

And since the sentence means seemed the kindest, the superlative phrase also has to be in instrumental:

• самая добрая → nominative
• самой доброй → instrumental

That is why Russian says:

казалась самой доброй

not казалась самая добрая.

No. Here самой is not the reflexive-like meaning herself. It is the instrumental feminine singular form of самый, which is used to make the superlative:

• добрый = kind
• самый добрый = the kindest

Because казаться requires the instrumental, both words change:

• самый добрый → самой доброй

So самой доброй means the kindest, not by herself.

Сделала is perfective past tense.

Russian uses the perfective here because the confession is presented as a single completed action: she made a confession.

• сделала = completed act
• делала = was doing / used to do / process, which would not fit as well here

So сделала неожиданное признание means she actually came out with the confession as a finished event.

Because признание is a neuter singular inanimate noun, and неожиданное agrees with it.

After сделала, this phrase is the direct object, so it is in the accusative case. But for neuter inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: неожиданное признание
• accusative: неожиданное признание

Same form, different function.

Та is in the nominative case because it is the subject of the main verb сделала.

The whole subject is:

Та, кто казалась самой доброй

And the main action is:

сделала неожиданное признание

So та is nominative because she is the one who performed the action.

Yes. Russian word order is flexible, and you could rearrange parts of the sentence for emphasis. But the original order is very natural and neutral.

This version first identifies the person:

Та, кто казалась самой доброй...

and then gives the surprising action:

...сделала неожиданное признание.

That order works especially well because it builds up the contrast between how she seemed and what she actually did.