What are some character traits of Laertes in Hamlet?

While Laertes is bold and rash as he crashes into Elsinore, calling Claudius "O thou vile king! (V,i,116), and while he is gullible in believing Claudius's intention are for Laertes to avenge himself against Hamlet, Laertes does have some redeeming characteristics.

For instance, just before Laertes wounds Hamlet with the rapier whose tip has been poisoned, he has a twinge of conscience as in an aside he says, "And yet it is almost against my conscience (V,ii,296).  Then, as he and Hamlet lie dying, Laertes asks Hamlet to forgive him as he forgives Hamlet:

Exchange forgiveness with me, noble Hamlet

Mine and my father's death come not upon thee,

'Nor thine on me! (V,ii, 308-310)

And, perhaps the most redeeming trait of Laertes is his devotion and love for his sister.  To Ophelia, Laertes gives sound advice when he learns that she loves Hamlet, cautioning her to remember that he is Prince of Denmark and may have to marry someone else, and to be careful with her affections:

...Perhaps he loves you now,

And now no soil nor cautel doth besmirch

The virtue of his will, but you must fear,

His greatess weighed, his will is not his own,

For he himself is subject to his birth.

He may not, as unvalued persons do,

Carve for himself, for on his choice depends

The safety and health of this whole state,

And therefore must his choice be circumscribed

Unton the voice and yielding of that body

Whereof he is the head.  Then if he says he loves you,

It fits your wisdom so far to believe it....[But]

If with too credent ear you list his songs,

Or lose your heart, or your chaste treasure open

To his unmastered importunity.

Fear it, Ophelia, fear it, my dear sister,

And keep you in the rear of your affection. (I,iii,14-34)

That he loves his sister dearly is also evidenced when Laertes jumps into her grave:

Lay her i' th'earth,

And from her fair and unpolluted flesh

May violets spring!  I tell thee, churlish priest,

A minist'ring angel shall my sister be

When thou liest howling (V,i,208-212)

Impetuous and at times rather foolish, Laertes is, nevertheless, forgiving and loving.

What are some character traits of Laertes in Hamlet?

In Shakespeare's Hamlet, Act 4.7 reveals two not-so-flattering character traits of Laertes.

Claudius hatches a plan, supposedly for Laertes to get revenge on Hamlet for the killing of Polonius, that will enable Laertes to get revenge without appearing to do so on purpose.  Claudius will arrange a fencing dual between Laertes and Hamlet, and

He [Hamlet], being remiss [careless],

Most generous, and free from all contriving,

Will not peruse [inspect] the foils, so that with ease,

Or with a little shuffling, you may choose

A sword unbated [unblunted], and in a pass of practice [treacherous thrust]

Requite him for your father.  (Act 4.7.132-137)

Laertes, with a "live" sword, instead of a protected one, can stab and kill Hamlet without appearing to have known the tip was unprotected. 

Laertes not only goes along with the plan, but adds his own touch, so to speak.

...I will do't,

And for that purpose I'll anoint my sword.

I bought an unction of a mountebank,

So mortal that but dip a knife in it,

Where it draws blood no cataplasm [poultice] so rare,

Under the moon, can save the thing from death

That is but scratched withal.... (Act 4.7.137-144)

Laertes will poison his sword tip, so just in case the wound he delivers to Hamlet is not mortal, the poison will kill him anyway. 

Laertes, then, is revealed to be gullible (he is being used by Claudius to get rid of Hamlet for Claudius' own reasons), as well as treacherous. 

In Hamlet, how does Laertes develop as a character throughout the play?

You asked quite a few questions about Laertes, and, since you may only ask one question per submission, I have chosen what I believe is the most signficant of the questions you asked.

Laertes spends a good deal of this play offstage and in France. We meet him briefly in Act One, scenes ii and iii.  He speaks in a very courtly manner in scene ii, when he asks Claudius' permission to return to France, and, already, there is a clear comparision drawn between him and Hamlet, who is belligerent and refuses to behave in a way that becomes the son of a king in this very public court scene.

In Act I, scene iii, Laertes is saying his farewells to his sister and father.  To Ophelia, he has cautionary words about Hamlet.  He tells her that she should be careful with her virginity around Hamlet, as he is a Prince and might be playing around with her, but with no intention of marrying her.  With his father, he endures Polonius' long speech about the ways he should conduct himself in the world, the sort of carefully planned, almost two-faced presentation of himself he should make.  This speech seems to make almost no impression on Laertes.  As soon as Polonius is done, he turns right back to Ophelia, reiterating what he has said about Hamlet.

In Act II, scene i, though Laertes does not appear, we learn much about how straight-laced and honorable Laertes is, because Polonius is sending Reynaldo to France to sully up his son's reputation a bit, so that he might seem more of a man of the world to his fellows.

Laertes re-appears in Act IV, scene v.  He has returned to Denmark for revenge.  He enters with every intention of murdering Claudius right then and there, assuming that the king was responsible for his father's death.  Again, this casts him in direct contrast to Hamlet, who has hemmed and hawed throughout the course of the first three acts of the play unable to avenge his own father's death.  Once Laertes is calmed down, he is apprised of Hamlet's guilt concerning Polonius' death.  Laertes immediately enters into a scheme with Claudius to kill Hamlet.  For Laertes, there is no hesitation towards revenge, and the witnessing of Ophelia's madness and her death only add fuel to his fire.

When Hamlet appears at Ophelia's grave, Laertes must be restrained to prevent him from killing Hamlet then and there.  Again, his decisive and knee-jerk impulse towards revenge contrasts with Hamlet's ponderous, overly-analytical hesitation towards his own revenge.

At the end of the play, Laertes realizes the role of villain that Claudius has played all along, and makes his peace with Hamlet as both are about to die.  Withdrawing his hatred of his father's murderer and going so far as to forgive him the act is a forgiveness that Hamlet is not willing to grant Claudius.

Since Laertes spends so much of the play offstage, I'm not sure how much development the audience can see from him, but he does serve the very important function of being a foil to Hamlet.  His actions are the exact opposite of Hamlet's response to the very similar circumstances in which they find themselves.

For more about Laertes and how he contrasts Hamlet, please follow the links below.

In Hamlet, how does Laertes develop as a character throughout the play?

At the start of the play Laertes is a concerned brother and a dutiful son.  He understands his place in court, as the son of Polonius, and fulfills the ritual of asking the king's permission to return to school.  He expresses a serious concern to Ophelia in regards to her relationship with Hamlet.  He makes a very valid point that no matter how Hamlet feels about Ophelia, in the end, princes don't get to make all their own choices, especially in regards to a wife.  Prince Hamlet may not "carve for himself, for on his choice rests the health of the whole nation."  He warns Ophelia to maintain her virtue, but he doesn't condescend to her (too much) and doesn't discount the feelings Hamlet and Ophelia have for each other.  As regards his father, he patiently listens to the laundry list of fatherly advice before heading back to school.

We don't see or hear from him again until the middle of Act 4, but when we do, we see a very different man.  He is furious over the death of his father and initially blames Claudius for not reigning in Hamlet in his craziness.  He is ready to punish Claudius.  After he is calmed, he then expresses his anger and outrage with Hamlet, even saying that he would "kill him in a church!"  Claudius uses this emotion to his advantage and manipulates him into the plot to lure Hamlet to a fencing match where they can use poisoned swords and wine to cause the death of Hamlet.  He is especially driven to this act when he sees how Ophelia has gone crazy, and later when he learns that she has let herself drown. 

While he begins the sword fight with a lot of verve, he is pretty quickly swayed to forgive Hamlet his past actions.  As the sword fight continues, he strikes at Hamlet with the poisoned sword in a moment of frustration, but once he knows he too is struck with poison he quickly reveals the whole plot so that Hamlet can punish the man who ultimately is responsible for all of this -- Claudius.  Leartes seems to have regained his conscience; at the very least he didn't want to die with that secret on his soul.

Leartes transforms from a gentle young man to a man of extreme passion and aggressive action.  Unfortunately those actions were not well thought out, and he is destroyed by those actions