King Henry VI
From the moment he first appears on stage, it is clear King Henry will have trouble reining in his feuding noblemen, most of whom are older relatives of his. Henry's speeches reveal him to be a pious and peaceable young man who lacks the experience (or the guile) to deal with his uncles' politicking. During Part 1 Henry is protected by his uncle Gloucester, but his innocence and vulnerability are apparent to allies and enemies alike.
Lord Talbot is a warrior's warrior: courageous, noble, and willing to undertake even the most desperate gambit if he thinks it will serve the English cause. Poignant scenes with his son John reveal his paternal affection. Feared by the French, Talbot is responsible for the major English victories in Acts 2 and 3. His death in Act 4 strikes a heavy blow to the English war effort.
Apart from Talbot, Gloucester is the closest thing to an unequivocal "good guy" in the play. He advises King Henry, his nephew, with an eye to England's best interests—even when the young king does not like what he has to say. Gloucester despises Winchester, whom he sees as a political schemer hiding behind the sanctity of the Church.
In the early acts the Bishop of Winchester is important mainly as a rival to Gloucester, who (rightly) sees him as worldly, ambitious, and not at all pious. In Act 5 Winchester is somewhat confusingly elevated to the rank of cardinal, even though he has previously been referred to by that title. He wields great power over the Church in England, but Gloucester and his allies try to limit his influence on secular politics.
The Duke of Somerset is a leading member of the House of Lancaster, which obtained control of the English throne by force two generations ago. In Act 2 he and Plantagenet quarrel about a personal matter, essentially forcing their fellow lords to pick sides. Somerset generally comes across as more petty and vindictive than Richard Plantagenet (later Duke of York). In Act 4 he refuses to provide aid to the besieged Lord Talbot because Richard has not specifically asked him to.
Richard Plantagenet first appears in Act 2 of the play, where his quarrel with Somerset pushes the country one step closer to civil war. Although he begins the play as a yeoman (i.e., not a knight or a nobleman), he eventually recovers the ancestral title of Duke of York, much to Somerset's dismay. He later replaces the deceased Bedford as Regent of France. Plantagenet's (later York's) growing feud with Somerset is one of the more important subplots of the play, though open war does not develop until Part 2. Of the two Plantagenet appears as more heroic than Somerset, who is selfish and underhanded in his feud with Plantagenet.
Joan la Pucelle
When Joan first makes herself known to the French Dauphin, she is seen as a literal godsend: a warrior-saint who will help France to throw off the shackles of English rule and end the war for good. Early in the play she helps the French to a notable victory at Orléans, but her later efforts are less successful. Only in Act 5 does the audience learn the true source of Joan's powers: diabolical magic. Joan ends the play in disgrace, captured by the English and led offstage to be burned as a witch.