Con el tiempo, mi amiga se ha vuelto menos tímida y más segura en clase.

Con el tiempo is a fixed expression that means over time, as time passes, or in the course of time.

Even though con usually means with, you should learn con el tiempo as a set phrase. In English, with the time would sound unnatural here, but in Spanish this is the normal expression.

Similar ideas:

• Con el tiempo, todo cambia. = Over time, everything changes.
• Con el tiempo, aprendí a hacerlo mejor. = In time, I learned to do it better.

Because this is the verb volverse, not plain volver.

• volver usually means to return or to come back
  • Ha vuelto a casa. = He/She has returned home.
• volverse means to become or to turn into in the sense of a change
  • Se ha vuelto más segura. = She has become more confident.

That se is part of the verb here. It is not optional.

A few common examples:

• Se ha vuelto muy paciente. = He/She has become very patient.
• Me he vuelto más organizado. = I have become more organised.

Because the sentence talks about a change over time, not just a current state.

• es tímida = she is shy (a general characteristic)
• está tímida = she is feeling shy / acting shy right now
• se ha vuelto menos tímida = she has become less shy

So volverse is useful when someone’s personality, attitude, or condition has changed.

Other verbs that can sometimes mean to become are:

• ponerse — often for changes in mood or temporary states
• hacerse — often for professions, ideology, or deliberate change
• convertirse en — to turn into / become something more dramatically

But in this sentence, volverse fits very naturally for a change in personality.

Both are possible, but they are used differently.

In Spain, the pretérito perfecto (se ha vuelto) is very common for something connected to the present or seen as part of a period that still matters now.

So:

• Mi amiga se ha vuelto menos tímida suggests a change that has happened and is relevant now.

By contrast:

• Mi amiga se volvió menos tímida uses the pretérito indefinido, which presents the change more as a completed past event.

In Peninsular Spanish, the perfect is especially common with ideas like:

• this week
• lately
• over time
• situations still connected to the present

Here volverse means to become.

It is often used for noticeable changes in a person’s character, behaviour, or condition. It can be negative, but it does not have to be.

Examples:

• Se ha vuelto antipático. = He has become unpleasant.
• Se ha vuelto más amable. = He has become kinder.
• Se ha vuelto loco. = He has gone crazy.

So in your sentence, it is completely natural and not negative:

• se ha vuelto menos tímida y más segura = she has become less shy and more confident

Because they describe mi amiga, which is feminine.

In Spanish, adjectives usually agree with the noun they describe in gender and number.

• mi amiga → feminine singular
• so: tímida, segura

If it were a male friend, you would say:

• Mi amigo se ha vuelto menos tímido y más seguro en clase.

If it were plural:

• Mis amigas se han vuelto menos tímidas y más seguras en clase.

Spanish often avoids repeating words that are already understood.

So:

• mi amiga se ha vuelto menos tímida y más segura

means:

• my friend has become less shy and more confident

You could think of it as:

• se ha vuelto menos tímida y se ha vuelto más segura

But repeating the verb would sound unnecessary. One verb can govern both adjective phrases.

This is very common in Spanish:

• Es inteligente y trabajadora.
• Está cansado pero contento.
• Se ha vuelto más abierta y menos insegura.

Segura here means confident or self-assured, which fits the sentence very well.

• seguro/a de sí mismo/a = self-confident
• segura en clase = confident in class

Confiado/a can sometimes mean confident, but very often it means overconfident, too trusting, or unsuspicious, depending on context.

So más segura is the safer and more natural choice here for a learner.

Examples:

• Ahora está más segura al hablar. = Now she is more confident when speaking.
• Es demasiado confiado. = He is too trusting / too confident.

Here en clase most naturally means in class — that is, during lessons or in the classroom context.

It can refer to:

• how she behaves during class
• how confident she is when participating in lessons

It does not necessarily focus on the physical room itself. It is more about the situation or setting.

Compare:

• Es muy callada en clase. = She is very quiet in class.
• Dejó el libro en la clase would sound odd for in the classroom; more naturally you would say en el aula if you mean the physical room.

Because menos and más can be used without que when you are not making an explicit comparison with another person or thing.

Here:

• menos tímida = less shy
• más segura = more confident

This means she is now less shy and more confident than before, but Spanish does not need to state than before.

You use que only when the second part of the comparison is expressed:

• Es menos tímida que antes. = She is less shy than before.
• Es más segura que su hermana. = She is more confident than her sister.

You could say similar things, but they do not sound exactly the same.

• se ha vuelto menos tímida = very natural for a change in personality or behaviour
• ha llegado a ser menos tímida = she has come to be less shy; this sounds more gradual or formal
• se ha hecho menos tímida is possible, but less natural here

For personality changes, volverse is one of the most idiomatic choices.

So for everyday Spanish, your original sentence is excellent:

• Con el tiempo, mi amiga se ha vuelto menos tímida y más segura en clase.