John's royal lineage is not in question, but what he can legally claim to possess based on that lineage is in doubt. The Bastard, on the other hand, is legally entitled to inherit the lands of his foster father. His father's deathbed will cannot move the law, which says that the offspring of a wife's affair is the legitimate son of her husband. Surprisingly, being a bastard child is not a barrier to inheritance; the Bastard can become a landed squire in place of his brother, an actual blood child of the Falconbridge line. Apparently being in the right position is vital to gaining possessions legally, even if one's lineage is in question. The Bastard is in the right spot to overturn his scandalous birth, but Arthur, whose lineage is in order, is not in the right place to claim the throne of England.
However, the Bastard turns down the inheritance, choosing to become a landless knight known as the bastard son of Richard the Lionhearted rather than a landholding gentry with the name of Falconbridge. His ambitions are larger than those of a mere landholder, and becoming a knight with the royal name Plantagenet pleases him more.
Unlike many of the characters in King John, the Bastard is not an actual historical figure, and in many ways he is less a coherent character than a set of theatrical functions. Shakespeare based him in part upon the vice figure, a mischievous allegorical character common in earlier English morality plays. The vice figure combined a commitment to evil with an intimacy with the audience and an alluring sense of fun. In asides and soliloquies, he denounces the failings of the royals while he gleefully announces his subscription to their self-interested schemes. However, later in the play the Bastard becomes one of the more responsible figures, proving himself an ethical center in a play largely without a rhetoric of positive values. The Bastard becomes the most vital and most interesting character in the play.
Lady Falconbridge arrives with the intention of defending her honor against the claims her son makes against her, first bringing up the play's concern with the uncertainty of biologically legitimate patriarchal succession. Hereditary descent from father to son requires wives to be sexually faithful to their husbands--but no father can ever be completely sure of their sons' paternity. The role of women, therefore, is necessary to hereditary lineage, but it is also a potential threat. This anxiety is later reflected in vicious exchanges between Constance and Eleanor when they accuse each other of infidelity. These mothers offer potential damage to their sons even after their lineages are assured, as we see through their micromanagement of the careers of John and Arthur, and the fact that both sons seem to weaken considerably after the deaths of their mothers.