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Originally William Thomas Kopp, he changed his name to Sylvester DeKeen.

Birth date from family Bible.

1900 U. S. Census for Baltimore City, Maryland:
Vol. 11, E.D. 76, Sheet 4, Line 86
Montford Avenue, Baltimore, house number 823
Charles Kopp, white, born June 1857, age 42, born in Maryland. Day laborer. Both parents born in Germany. He owned his house, which was mortgaged. Married 18 years
Minnie Kopp, wife, born June 1862 in Maryland, age 37. She had had 10 children, of whom 5 were living. Both parents born in Germany
William Kopp, son, born December 1881 in Maryland, age 18, boiler maker
Charles Kopp, son, born July 1886 in Maryland, age 13, at school
May Kopp, daughter, born October 1891 in Maryland, age 8, at school
Julia Kopp, daughter, born May 1895 in Maryland, age 5
John R. W. Kopp, son, born December 1897 in Maryland, age 2
All could read and write.

On 12 April 1902, The Baltimore Sun posted:
Celebrates Twelfth Anniversary.
The members of the Patterson Lodge, No. 39, Shield of Honor, celebrated the twelfth anniversary of its organization with a banquet last night at Lewis’ Hall, Ann and Jefferson streets. Covers were laid for 100. The decorations were suggestive of patriotism. John W. Meeks, supreme treasurer; James J. McNamara, grand chaplain, and William F. Edwards, junior supreme master, responded to toasts. The banquet committee consisted of Gustav G. Siegel, chairman; Edward Giudice, secretary; Wellington S. Dawson, treasurer; Henry O. Kommalan, Wm. T. Kopp, James G. Dobson, Robert L. Donaldson.
[The Baltimore Sun, 12 April 1902, page 12.]

On 10 October 1902, The Baltimore Sun published in its Marriage Licenses column for Baltimore:
KOPP—RUMBEL.—William T. Kopp, 22, 17 North Durham street; Marguerite A. Rumbel, 18.
[The Baltimore Sun, 10 October 1902, page 7.]

On 30 September 1912, The Baltimore Sun published a list of Clerks of Elections, including, in the Sixth Ward, 10th Precinct, republican William T. Kopp of 30 N. Durham Street.
[The Baltimore Sun, 30 September 1912, page 5.]

On 15 May 1918, The Baltimore Sun published:
Several Granted Divorces
[Yesterday] Absolute divorces were granted to Margaret A. Kopp, 30 North Durham street, from William T. Kopp, with the custody of their two children…
[The Baltimore Sun, 15 May 1918, page 5.]

Marriage date to Mary from family Bible.

On 12 September 1918, Sylvester Winchester DeKeen registered for the World War I draft, giving his full name as Sylvester Winchester DeKeen; residing at 107 N. Streeper, Balto, Md; age 41; born 25 February 1877; white; native born; master mechanic in copper for Balto Copper Wks of 4th St & 4th Ave Canton Md; nearest relative Mary DeKeen of 107 N. Streeper, Balto, Md; medium height, medium build; blue eyes, dark brown hair; no disabilities. [World War I Draft Registration Card, roll 1665871.]

1920 U.S. Census, Baltimore City, Maryland
6th Ward, Enumeration District 71, page 11A
107 Streeper
DeKeen, Sylvester, head, owned, mortgaged, male, white, 44, married, can read and write, born in Maryland, both parents born in Maryland, speaks English, laborer in smelting company, wage worker
DeKeen, Marie, wife, female, white 44, married, can read and write, born in Maryland, both parents born in Maryland, speaks English, no occupation
DeKeen, Chas, son male, white, 16, single, in school, can read and write, born in Maryland, both parents born in Maryland, speaks English, no occupation

The 1921 Baltimore City Directory lists on page 685:
De Keen, Sylvester W machbd [machhd?] h 107 n Streeper

1929 Baltimore City Directory, page 539:
DeKeen S W Heating & Plumbing Supply Co (Sylvester W DeKeen) 3000 Hudson

1936 Baltimore City Directory, page 329:
DeKeen Sylvester W (Mary B) plmbg supplies 3000 Hudson h do

1937 Baltimore City Directory, page 308:
DeKeen Sylvester W (Mary) plmbr supplies 3000 Hudson h do

Sylvester DeKeen was mentioned in the 3 February 1938 will of his mother Minnie Donaldson. He was to receive one-seventh of her real and personal estate. The will was proved on 5 August 1939. [Baltimore City, Maryland, Wills, J.H.B. 193, pages 277–279.]

On 1 July 1939, in the settlement of the estate of his mother Minnie Donaldson, Sylvester Dekeen received one-seventh of the estate. [Baltimore City, Maryland, Administration Accounts J.H.B. No. 358, pages 91–93.]

1940 U.S. Census, Baltimore City, Maryland
1st Ward, Enumeration District 4-35, Sheet 11
3000 Hudson Street, household 303, owned, value $3,500, not on a farm
DeKeen, Sylvester W., head, male, white, 58, married, not in school, highest level of education 5th grade, born in Maryland, living in same house in 1935, at work week of March 24–30, worked 54 hours that week, hardware store owner, works on own account, 52 weeks employed in 1939, has additional income
DeKeen, Mary B., wife, female, white, 61, married, not in school, highest level of education 5th grade, born in Bohemia, naturalized, living in same house in 1935, not at work, not seeking work, engaged in housework, has no additional income

On 25 April 1942, Sylvester Windsor DeKeen completed a World War II draft registration card, giving his place of residence as 3000 Hudson Street, Baltimore, Maryland; mailing address the same, telephone Wo. 7883; 61 years of age, born 30 December 1881 in Baltimore, Maryland; name of person who will always know address as Lena Schimenuk, 2844 Hudson Street, sister-in-law; owns own business at 3000 Hudson Street, Baltimore, Maryland; race white, height 5’5”, weight 130, brown eyes, brown hair, ruddy complexion.

On 4 March 1944 at 12:15 a.m., Sylvester DeKeen died at St. Joseph’s Hospital, 1400 N. Caroline Street, Baltimore, Maryland. He was male, white, widowed, wife was Mary (nee Schimunek). He was born in 1876, aged 68 years, born in Maryland, occupation plumber, owner or his own business. Residence was 3000 Hudson Street, Baltimore. Father unknown, mother unknown. Informant was Milton Kopp (son) of 1142 Neucomb, Armistead Gardens. Attended by physician from 3 March to 4 March. Immediate cause of death was Acute Cardiac Decompensation[?] due to arteriosclerosis C.U. Disease. Buried on 6 March 1944 at First United Evangeical Cemetery, 6115 O’Donnell Street, Baltimore City. Funeral director was Charles F. Schimunek of 2601 E. Madison Street. [Baltimore City, Maryland, Certificate of Death G14760.]

A death notice stated:
DE KEEN.—On March 4, 1944, SYLVESTER, of 3000 Hudson street, beloved husband of the late Mary (nee Schimunek). Funeral from the Schimunek Funeral Home, 2601 East Madison street, on Monday at 1 P. M. Interment in First United Evangelical Church Cemetery. [The Sun (Baltimore), 5 March 1944, page 10-A; 6 March 1944, page 14.]