If you are subpoenaed to court, you either need to appear or have the subpoena withdrawn, either by having the party that issued it withdraw it or having the court quash it. Failing to respond to a subpoena can have serious consequences.
Rules for issuing and serving subpoenas vary from state to state. Some states require the person asking for the subpoena to pay the person being subpoenaed for expenses and a witness fee. Not being a Delaware lawyer, I do not know what the rules are in Delaware (and you should consult a Delaware lawyer or ask the court that issued the subpoena).
If you think you might be subpoenaed as a character witness and you do not want to serve as such a witness, you might tell your friend's lawyer that. He may not want to subpoena a potentially hostile witness. However, if a subpoena is served on you, you should appear in court unless the court gives you permission not to appear.
Generally, most character witnesses were not present at the scene of the crime, but that doesn't mean they don't have relevant testimony to offer. If subpoenaed as a witness, failing to appear is a form of contempt for which you can be criminally charged.
Many states permit the issuing of a body attachment, which permits law enforcement to arrest a person who fails to appear pursuant to a subpoena and place them in jail until their testimony is needed.
A person who believes they shouldn't be required to appear by subpoena can file a motion to quash the subpoena. The judge will then decide if the subpoena is valid.
A subpeona is a court order to appear. You can only refuse to speak if you have a privilege you are exercising such as the martial privilege or the fifth amendment.
My standard disclaimer: I am not offering legal advice, assume I do
not know the law in your state or at all for that matter and that I am
just making suggestions for starting points for when you do speak with
an attorney. Do NOT rely on anything I write and contact a lawyer in
your area immediately after reading my posting.