La jefa de mi hermana es seria, pero en persona resulta bastante maja.

Because the boss is female.

• jefe = boss (male, or grammatically masculine)
• jefa = boss (female, grammatically feminine)

Spanish normally makes job titles and professions agree in gender with the person:

• el jefe → male boss
• la jefa → female boss
• el médico / la médica → (male / female doctor)

So la jefa de mi hermana literally means the (female) boss of my sister.

Spanish doesn’t use an ’s structure for possession. Instead, it uses:

[possessed thing] + de + [owner]

So:

• la jefa de mi hermana = my sister’s boss
• el coche de mi padre = my father’s car
• los amigos de Marta = Marta’s friends

You can say mi jefa (my boss), but as soon as you specify whose boss (my sister’s boss, your friend’s boss, etc.), you use de:

• la jefa de mi hermana
• el jefe de mi amigo

Spanish uses the definite article a lot more than English, especially with specific people and professions.

When you refer to a specific boss, doctor, teacher, etc., you normally include el / la:

• La jefa de mi hermana es seria.
• El profesor de mi hijo es estricto.
• La médica de mi madre es muy buena.

Omitting the article (∗Jefa de mi hermana es seria) is incorrect in standard Spanish.

Adjectives in Spanish agree with the gender and number of the noun they describe.

• la jefa → feminine singular
• So the adjective must also be feminine singular: seria

Other examples:

• el jefe es serio
• las jefas son serias
• los jefes son serios

Seria often means more than just “not joking”. In this context it suggests a person who is:

• reserved
• formal
• not very smiley
• maybe a bit strict

So es seria is more like:

• “she’s a serious / rather strict person”
• “she’s not very playful, she’s quite reserved”

It’s a character trait, not just a temporary mood.

Both are grammatically correct, but they don’t mean exactly the same:

• es seria → a general, stable trait: “she is (in general) a serious person”
• resulta bastante maja → how she turns out to be / comes across when you actually meet her.

resultar + adjetivo in this sense means:

• “to turn out (to be)”
• “to come across as / end up seeming”

So the contrast is:

• In general / by reputation: es seria
• But when you actually meet her: (en persona) resulta bastante maja

If you said:

• …pero en persona es bastante maja,
  you would be stating both as simple facts, with less emphasis on the surprise or contrast in impression.

The verb is resultar, and here it does not mean “to result” in the technical or mathematical sense.

With adjectives, resultar often means:

• to turn out to be
• to come across as / to seem

Examples:

• El examen resulta fácil. → The exam turns out to be easy.
• Tu amigo resulta muy simpático. → Your friend turns out to be very nice.

So resulta bastante maja ≈ “she turns out to be quite nice” / “she actually comes across as quite nice”.

In (especially Peninsular) Spanish, majo / maja is a very common, informal/colloquial word.

Here, maja means something like:

• nice
• pleasant
• friendly
• easy to get along with

Rough comparisons:

• simpática → nice, friendly, likeable
• amable → kind, polite, helpful
• maja → nice, pleasant, down‑to‑earth (colloquial, very typical in Spain)

So bastante maja ≈ “pretty nice / really nice (as a person)” in casual speech.
In Latin America, majo/maja is much less common or can sound peninsular.

Yes, in Spain majo/maja can sometimes suggest that someone is fairly attractive, but usually in a soft, everyday way, not in a strong “sexy” sense.

However, in your sentence, combined with seria and en persona, the natural reading is about her personality (nice, pleasant), not about her looks.

Context will tell you whether majo/maja is about:

• character (most common in this kind of sentence), or
• appearance (e.g. Es un chico majo, está bastante bien could imply he’s nice and also reasonably good‑looking).

Bastante has two main uses:

1. “enough / sufficient” (usually before a noun)

   • Tenemos bastante comida. → We have enough food.

2. “quite / rather / pretty” (before an adjective or adverb)

   • Es bastante maja. → She’s quite nice / pretty nice.
   • Habla bastante rápido. → He speaks quite fast.

In bastante maja, it’s the second meaning: an intensifier like quite / pretty / fairly.

En persona literally means “in person”. Here it implies:

• when you actually meet her face to face,
• not just by reputation, on paper, or from a distance.

So the contrast is:

• From what you might think: she es seria (is serious)
• But when you meet her in real life: she resulta bastante maja (turns out to be quite nice).

It’s similar to saying in English:

• “She’s serious, but in person she’s actually pretty nice.”

You could say:

• La jefa de mi hermana es seria, aunque en persona resulta bastante maja.

Both pero and aunque express contrast, but:

• pero = but → simple opposition between two facts.
• aunque = although / even though → often sounds a bit more concessive, like “despite that”.

In everyday spoken Spanish, pero is more common in this kind of sentence and sounds very natural and neutral.
Aunque would sound a bit more formal or literary here, but still correct.