Re-reading ch. 4, gave me an idea of what Bianca meant in her chatterings to Matilda on Isabella.

Bianca, 'No, no madam; my lady Isabella is of another-guess mould than you take her for'. (ch.2) Not just gossip ?

My impression of Isabella is she is no saint yet not impious. She feels genuine sympathy with Manfred's wife and daughter. Warm hearted, with a youthful will Not to be victim to a tyrant. She is of a naturally cheerful dispostion (says Matilda, ch. 2) and lively.
It is to Isabella's credit, she admits Matilda is the one Theodore admires. Despite some relutance she concedes him quite amicably.

Isabella was right to warn Hippolita and Matilda of Manfred. They were too credulous and obedient to his will whilst she had seen first-hand the depravity of Manfred.

'What ruin has thy inadverent goodness been preparing for thyself, for me, and for Matilda !'.

Matilda seems more vague to pick up.
Saintly, gentle and kindly. A noble daughter of her Times- she practises virues of piety and filial obediance.
Her converse with Bianca shows she is not superstitious and seeks rational answers yet she's fascinated by Theodore's resemblance to the portrait of Alfonso.
Despite her father, and not being so heroic she takes the opportunity to perform the selfless act of freeing Theodore from the black tower.

Isabella & Matilda aren't exactly a 'Thelma and Louise' team ;-)
(in the film the heroines were in cross country flight from men ! )-- yet I begin to like them.
Like Catherine, Isabella shows some pluck ! She was wary of Manfred's friendship as Catherine was of the General.
Gosh ! Like Catherine, Isabella hasn't come close to a faint. ;)