Gertrude is a shadowy character with little substance on which to hang a characterization. We can examine her through what others say about her more than through what she says.
That she is "th'imperial jointress" to the throne of Denmark indicates that she wields some power and suggests that Claudius' decision to marry her had political implications. Yet Hamlet indicts all women by calling her fickle — "frailty, thy name is woman." We see through Hamlet the picture of a woman who one day lived obediently and in the shadow of one king to whom she was devoted. The next day she allies herself in love and politics with the polar opposite of the man she formerly called husband.
The most haunting questions about Gertrude's character revolve around whether she knows that Claudius is a criminal. Is she merely a dependent woman who needs to live through her man? Is she a conniving temptress who used her power to conspire with Claudius to kill King Hamlet and usurp Prince Hamlet's ascendancy?
No textual references are conclusive. The ghost of King Hamlet calls her his "most seeming virtuous queen." He entreats Hamlet to "Leave her to Heaven / And to those thorns that in her bosom lodge / To prick and sting her." These words could imply that she has reason to be guilty, that she is not blameless. Later, the ghost implores Hamlet to comfort her. "But look, amazement on thy mother sits. / Oh step between her and her fighting soul." Again, he waxes protective of her but implies that she has some reason to be spiritually conflicted.
When Rosencrantz and Guildenstern arrive at Elsinore, she tells them that they have been sent for because of the way Hamlet "hath talked of you," and she promises them compensation fit for " a king's remembrance." She exhibits apparent sincerity in her concern for Hamlet, and yet, even after Hamlet has told her what he knows about Claudius, even after he has shared his fears of the trip to England, even after Hamlet has clearly proven that something is rotten in the state of Denmark, she never opposes Claudius to protect Hamlet. Unless, as some critics believe, she drinks the poisoned wine as an act of maternal protectiveness. Does she know the wine is poisoned? When "the Queen carouses to thy fortune, Hamlet" is she deliberately drinking to prevent Hamlet's death?
If Gertrude has overheard Claudius and Laertes plotting, she would know all. If she is in Claudius' confidence, she would be complicit with all his conspiracies. Though Claudius professes love and admiration for Gertrude, he never confides to anyone the extent of their relationship. Gertrude describes her love for Hamlet when she asks him not to return to Wittenberg. When she shares with Ophelia her hope that the young woman would have married her Hamlet, she divulges her wish for his happiness. However, she never declares any kind of emotion for Claudius, either positive or negative.
Ultimately, Gertrude's character remains malleable. In the hands of an astute actor and a clever director, she can come across as either Claudius' co-conspirator or Hamlet's defender. Either interpretation works, if built substantially.